Separation membrane

ABSTRACT

A problem to be solved by the present invention is to provide a separation membrane having excellent separation performance, having high membrane strength and high permeation performance, and mainly including a cellulose-based resin. The present invention is concerned with a separation membrane including a cellulose ester, having, in the interior thereof, voids each having a specified structure, and having a tensile elasticity of 1,000 to 6,500 MPa.

TECHNICAL FIELD

The present invention relates to a separation membrane having excellentseparation performance and having high membrane strength and highpermeation performance.

BACKGROUND ART

Cellulose-based resins have been widely used as separation membranesincluding water treatment membranes, because they have permeationperformance due to their hydrophilicity and have chlorine resistanceperformance of being strong against chlorine bactericides.

For example, Patent Document 1 discloses a hollow fiber membraneobtained by melt-spinning a mixture obtained by mixing a water-solublepolyhydric alcohol having an average molecular weight of 200 to 1,000with cellulose diacetate.

In addition, Patent Document 2 discloses a hollow fiber membraneobtained by discharging from an arc-shaped nozzle a solution obtained bymixing N-methyl-2-pyrrolidone, ethylene glycol, and benzoic acid withcellulose triacetate, and immersing it in a coagulating bath includingN-methyl-2-pyrrolidone/ethylene glycol/water, followed by water washingand heat treatment.

BACKGROUND ART DOCUMENT Patent Document

Patent Document 1: JP-A-51-70316

Patent Document 2: JP-A-2012-115835

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The hollow fiber membrane obtained by the technique described in PatentDocument 1 described above shows high permeation performance, but onlyrealizes low separation performance. In addition, there is a problem inproduction that spinning cannot be performed at a high draft, becauseyarn breakage is likely to occur during the melt spinning, which causesinsufficient membrane strength of the resulting hollow fiber membrane.

The hollow fiber membrane obtained by the technique of Patent Document 2has excellent separation performance and permeation performance, but haslow membrane strength.

In view of such a background of the conventional techniques, an objectof the present invention is to provide a separation membrane having highmembrane strength and high permeation performance, and including, as amain component thereof, at least one compound selected from the groupconsisting of a cellulose ester, a polyamide, and a polyester.

Means for Solving the Problems

In order to solve the above-described problem, the present inventorsmade extensive and intensive investigations. As a result, it has beenfound that it is able to provide a separation membrane having excellentseparation performance and having high membrane strength and highpermeation performance when it has an internal structure including apredetermined tensile elasticity and specified voids, leading toaccomplishment of the present invention.

Namely, the present invention has any one of the followingconfigurations.

(1) A separation membrane including, as a main component thereof, atleast one compound selected from the group consisting of a celluloseester, a polyamide, and a polyester,

in which the separation membrane has, in an interior thereof, aplurality of voids each having a depth (D₁) of 10 nm or more and 500 nmor less, a length (L₁) of 30 nm or more, and a ratio L₁/D₁ of the lengthto the depth in a range of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(2) A separation membrane including, as a main component thereof, atleast one compound selected from the group consisting of a celluloseester, a polyamide, and a polyester,

in which the separation membrane has, on at least one surface thereof, aplurality of grooves each having a length (L₂) of 30 nm or more, a width(W₂) of 5 nm or more and 500 nm or less, and a ratio L₂/W₂ of the lengthto the width in a range of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(3) A separation membrane including at least a first layer and a secondlayer,

in which the first layer includes, as a main component thereof, at leastone compound selected from the group consisting of a cellulose ester, apolyamide, and a polyester,

the first layer has, in an interior thereof, a plurality of voids eachhaving a depth (D₁) of 10 nm or more and 500 nm or less, a length (L₁)of 30 nm or more, and a ratio L₁/D₁ of the length to the depth in arange of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(4) A separation membrane including at least a first layer and a secondlayer,

in which the first layer includes, as a main component thereof, at leastone compound selected from the group consisting of a cellulose ester, apolyamide, and a polyester,

the separation membrane has, on at least one surface thereof, aplurality of grooves each having a length (L₂) of 30 nm or more, a width(W₂) of 5 nm or more and 500 nm or less, and a ratio L₂/W₂ of the lengthto the width in a range of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(5) A membrane module including any one of the above-describedseparation membranes.

Advantage of the Invention

When the membrane of the present invention has, in the interior thereof,a plurality of voids each having a depth (D₁) of 10 nm or more and 500nm or less, a length (L₁) of 30 nm or more, and a ratio L₁/D₁ of thelength to the depth in a range of 2 or more, there gives rise to such aneffect that a substantial membrane thickness decreases, and a waterpermeation amount increases.

In addition, when the membrane of the present invention has a tensileelasticity of 1,000 to 6,500 MPa, there gives rise to such an effectthat a high membrane strength is exhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an internal structure and a surfacestructure of a separation membrane according to an embodiment of thepresent invention.

MODE FOR CARRYING OUT THE INVENTION

The present inventors made extensive and intensive investigationsregarding the above-described problem, namely a separation membranehaving excellent separation performance and having high membranestrength and high permeability. As a result, they have successfullysolved such a problem by a membrane having an internal structureincluding specified voids and having a tensile elasticity of 1,000 to6,500 MPa.

Specifically, the present invention provides a separation membranehaving an internal structure including specified voids.

Specific embodiments of the present invention are hereunder described.The separation membrane of the present invention may contain a liquid,such as water, etc., therein in order to retain its shape. However, inthe following description, such a liquid for retaining the shape is notconsidered as a constituent element of a hollow fiber membrane.

1. Separation Membrane (1) First Embodiment: Separation Membrane (FirstLayer)

The separation membrane (hereinafter also referred to as a membrane) ofthe present invention includes, as a main component thereof, at leastone compound selected from the group consisting of a cellulose ester(A), a polyamide (E), and a polyester (F). Namely, the main componentmay be a cellulose ester, a polyamide, or a polyester, or a mixture oftwo kinds or a mixture of three kinds of these compounds. The term“mixture of two kinds or mixture of three kinds of these compounds”hereinafter refers to simply as a mixture thereof. Such compound andmixture can also be expressed as a resin or a polymer.

The term “including as a main component” as referred to in the presentinvention means that a sum total of contents of at least one compoundselected from the group consisting of the cellulose ester (A), thepolyamide (E), and the polyester (F) is 70% by weight or more. In theseparation membrane of the present invention, the sum total of contentsof at least one compound selected from the group consisting of thecellulose ester (A), the polyamide (E), and the polyester (F) ispreferably 80% by weight or more, and more preferably 90% by weight ormore.

(1-1) Main Component

(1-1-1) Cellulose Ester (A)

Specific examples of the cellulose esters (A) include cellulose acetate,cellulose propionate, cellulose butyrate, and a cellulose-mixed ester inwhich 3 hydroxyl groups present in a glucose unit of cellulose areblocked with 2 or more types of acyl groups.

Specific examples of the cellulose-mixed esters include, for example,cellulose acetate propionate, cellulose acetate butyrate, celluloseacetate laurate, cellulose acetate oleate, and cellulose acetatestearate.

Each cellulose-mixed ester exemplified has acetyl groups and other acylgroups (for example, a propionyl group, a butyryl group, a lauryl group,an oleyl group, a stearyl group, etc.). It is preferred that the averagedegrees of substitution of the acetyl group and other acyl groups in thecellulose-mixed ester satisfy the following formulae. The term “averagedegree of substitution” refers to the number of hydroxyl groups to whichthe acetyl group and other acyl groups, respectively, are chemicallybonded, among 3 hydroxyl groups present per glucose unit of thecellulose.1.0≤{(Average degree of substitution of acetyl group)+(Average degree ofsubstitution of other acyl groups)}≤3.00.1≤(Average degree of substitution of acetyl group)≤2.60.1≤(Average degree of substitution of other acyl groups)≤2.6

When the above-mentioned formulae are satisfied, the membrane achievingboth the separation performance and the permeation performance isrealized. Further, when the above-mentioned formulae are satisfied, goodthermal flowability of a membrane forming raw material is realizedduring melt spinning, in the production of the separation membrane.

The separation membrane may contain either only one kind of thecellulose esters (A) or two or more kinds thereof.

In addition, in the separation membrane, specifically, it is preferredto contain particularly at least one of cellulose acetate propionate andcellulose acetate butyrate, among the cellulose esters (A) describedabove as the specific examples. The separation membrane having highseparation performance and high permeation performance is realized bycontaining such cellulose ester.

The weight average molecular weight (Mw) of the cellulose ester (A) ispreferably 50,000 to 250,000. When Mw is 50,000 or more, thermaldecomposition of the cellulose ester (A) during melt spinning issuppressed, and the membrane strength of the separation membrane canreach a practical level. When Mw is 250,000 or less, the melt viscositydoes not become excessively high, and therefore, stable melt spinningbecomes possible.

Mw is more preferably 60,000 to 220,000, and still more preferably80,000 to 200,000. The weight average molecular weight (Mw) is a valuecalculated by GPC measurement. A calculation method thereof is describedin detail in Examples.

(1-1-2) Polyamide (E)

Examples of the polyamide (E) include various polyamides obtainedthrough ring-opening polymerization of various lactams, polycondensationof various diamines and various dicarboxylic acids, polycondensation ofvarious aminocarboxylic acids, and so on; and copolymerized polyamidesthrough a combination of these ring-opening polymerization andpolycondensation. Specific examples of the above-described polyamidesand copolymerized polyamides may include nylons such as nylon 6, nylon66, nylon 610, nylon 46, nylon 612, nylon 11, nylon 12, a nylon 6/12copolymer (copolymer of ε-caprolactam and laurolactam), a nylon 6/66copolymer (copolymer of ε-caprolactam and a nylon salt ofhexamethylenediamine.adipic acid), but should not be construed as beinglimited thereto. In addition, two or more kinds of these polyamides canalso be kneaded and used.

The separation membrane may contain either only one kind of thepolyamide (E) or two or more kinds thereof.

In addition, in the separation membrane, specifically, it is preferredto contain particularly at least one of nylon 6 and nylon 66, among thepolyamides (E) described above as the specific examples. The separationmembrane having high separation performance is realized by containingsuch polyamide (E).

A weight average molecular weight (Mw) of the polyamide (E) ispreferably 10,000 to 1,000,000. Mw of 10,000 or more is preferred fromthe standpoint that thermal decomposition during melt spinning can besuppressed, and from the standpoint that the membrane strength of theseparation membrane can reach a practical level. Mw of 1,000,000 or lessis preferred from the standpoint that the matter that the melt viscositybecomes excessively high can be suppressed, so that stable melt spinningmay be achieved. Mw is more preferably 20,000 to 900,000, and still morepreferably 30,000 to 800,000.

In the polyamide, the bond formed through polymerization is an amidebond, and in particular, even in the case when it comes into contactwith an alkali, breakage of the main chain is hardly generated, andhence, the polyamide is preferred from the standpoint of having goodtolerance to the alkali.

(1-1-3) Polyester (F)

Examples of the polyester (F) include polyesters having a glycol moietyand a dicarboxylic acid moiety, and polylactic acid-based polymers. Theseparation membrane may contain either only one kind of the polyester(F) or two or more kinds thereof.

With respect to the polyester having a glycol moiety and a dicarboxylicacid moiety, from the viewpoint of permeability, the glycol moiety ispreferably a glycol having a carbon number of 18 or less, morepreferably a glycol having a carbon number of 10 or less, and still morepreferably a glycol having a carbon number of 5 or less. the carbonnumber of the glycol moiety of 2 or more is preferred from the viewpointof enhancing durability against alkaline hydrolysis. Specific examplesthereof include aliphatic dihydric alcohols such as ethylene glycol,1,2-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,2-methyl-1,3-propanediol, neopentyl glycol, 1,5-pentanediol,1,6-hexanediol, 2,2-diethyl-1,3-propanediol,2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-1,3-hexanediol, 1,9-nonanediol,1,10-decanediol, 2-butyl-2-ethyl-1,5-propanediol, 1,12-octadecanediol;and polyalkylene glycols such as dipropylene glycol. These glycols maybe used either alone or in combination of two or more kinds thereof.

Examples of the dicarboxylic acid moiety include terephthalic acid,isophthalic acid, and naphthalene dicarboxylic acid, but should not beconstrued as being limited thereto. When terephthalic acid is used, itbecomes possible to obtain a separation membrane having excellentmechanical characteristics and having excellent handling properties,because it becomes possible to enhance crystallinity of the resin. Whenisophthalic acid is used, good water permeability can be obtained,because excessive crystallization can be suppressed. Even when thedicarboxylic acid is used either alone or in combination of two or morekinds thereof, the effects of the present invention can be exhibitedwithout any problems.

In addition, the polyester having a glycol moiety and a dicarboxylicacid moiety may be copolymerized with other polymer (namely, acopolymerization component) within a range where characteristics thereofare not significantly altered. As the copolymerization component,5-(alkali metal) sulfoisophthalic acids such as 5-sodiumsulfoisophthalic acid, etc.; or polyvalent carboxylic acids other thanthe above-described dicarboxylic acids, such as ethanetricarboxylicacid, propanetricarboxylic acid, butanetetracarboxylic acid,pyromellitic acid, trimellitic acid, trimesic acid,3,4,3′,4′-biphenyltetracarboxylic acid, ester-forming derivativesthereof, etc., can also be used. In particular, the use of 5-sodiumsulfoisophthalic acid as the copolymerization component is preferredfrom the standpoint that the hydrophilicity of the polymer can beenhanced, thereby enhancing the water permeability.

Examples of the polyester having the above-described glycol moiety andthe above-described dicarboxylic acid moiety include polyethyleneterephthalate, polyethylene isophthalate, polyethylene naphthalate,polybutylene terephthalate, and a copolymer of polyethyleneterephthalate with 5-sodium sulfoisophthalic acid.

A weight average molecular weight (Mw) of the polyester having a glycolmoiety and a dicarboxylic acid moiety is preferably 10,000 to 1,000,000.Mw of 10,000 or more is preferred from the standpoint that thermaldecomposition during melt spinning can be suppressed, and from thestandpoint that the membrane strength of the separation membrane canreach a practical level. In addition, Mw of 1,000,000 or less ispreferred from the standpoint that the matter that the melt viscositybecomes excessively high can be suppressed, so that stable melt spinningmay be achieved. Mw is more preferably 20,000 to 900,000, and still morepreferably 30,000 to 800,000.

(1-2) Plasticizer (B)

The separation membrane of the present invention may contain aplasticizer (B). The plasticizer (B) may remain in the separationmembrane or may be eluted from the separation membrane into water afterthe cellulose ester, the polyamide, or the polyester, or the mixturethereof as the main component has been plasticized during melt spinning.When the plasticizer (B) is eluted into water, traces formed by elutionof the plasticizer become fine pores in the membrane to improve thepermeation performance.

The plasticizer (B) is not particularly limited, as long as it is acompound which thermoplasticizes the cellulose ester (A), the polyamide(E), or the polyester (F), or the mixture thereof as the main component.Further, not only one kind of the plasticizer but also two or more kindsof the plasticizers may be used in combination.

As the plasticizer (B), preferred is a polyhydric alcohol-basedcompound. Specifically, examples thereof include polyalkylene glycols,glycerin-based compounds, caprolactone-based compounds, and derivativesthereof. Of these, the polyalkylene glycols have good compatibility withthe cellulose esters, and exhibit thermoplasticity even when added insmall amounts. The polyalkylene glycols are therefore preferred, interms of suppressing a decrease in the membrane strength due to theplasticizers and the fact that the pores formed after elution becomesfine to make it possible to achieve both the separation performance andthe permeation performance.

Examples of the plasticizer for the polyamide and the polyester includepolyalkylene glycols, polyvinylpyrrolidone, and copolymer-basedplasticizes thereof. Specific examples thereof includepolyvinylpyrrolidone, and a polyvinylpyrrolidone-vinyl acetatecopolymer, each having a weight molecular weight (Mw) of 3,000 to900,000.

Specific examples of the polyalkylene glycol include polyethyleneglycol, polypropylene glycol, and polybutylene glycol, each having aweight average molecular weight of 400 to 2,000.

(1-3) Additive (C)

The separation membrane may contain an additive (C).

Specific examples of the additive (C) include phthalic acid estercompounds, aliphatic dibasic acid esters, polyester-based compounds,epoxy-based compounds, phosphoric acid ester-based compounds andtrimellitic acid ester-based compounds, polyalkylene glycol-basedcompounds such as polyethylene glycol, polypropylene glycol,polybutylene glycol, etc., and plasticizers such as glycerin-basedcompounds, caprolactone-based compounds, and derivatives thereof. Thesecan be used either alone or in combination thereof. The additive (C) ispreferably diglycerin oleate or polyethylene glycol.

(1-4) Antioxidant (D)

The separation membrane may contain an antioxidant (D). The antioxidantis preferably a phenol-based antioxidant, a phosphorus-basedantioxidant, or the like. In the case where the separation membranecontains a cellulose ester, in particular, the antioxidant is preferablya phosphorus-based antioxidant, and especially preferably apentaerythritol-based compound. In the case where the antioxidant iscontained, thermal decomposition during melt spinning is suppressed. Asa result, it becomes possible to improve the membrane strength and toprevent the membrane from being colored.

(1-5) Membrane Shape

Although the shape of the separation membrane is not particularlylimited, a membrane having a hollow fiber shape (hereinafter alsoreferred to as a hollow fiber membrane) or a membrane having a plannershape (hereinafter also referred to as a flat membrane) is preferablyadopted. Of these, the hollow fiber membrane is more preferred, becauseit is possible to be efficiently filled in a module, thereby being ableto enlarge an effective membrane area per unit volume of the module. Thehollow fiber membrane is a fibrous membrane having a hollow.

From the viewpoint of improving the permeation performance, a thicknessof the separation membrane is preferably 100 μm or less, more preferably50 μm or less, and still more preferably 30 μm or less. Meanwhile, fromthe viewpoint of obtaining a sufficient membrane strength, the thicknessof the separation membrane is preferably 2 μm or more, more preferably 3μm or more, and still more preferably 4 μm or more.

In the case of the hollow fiber membrane, from the viewpoint ofachieving both the effective membrane area at the time when filled inthe module and the membrane strength, an outer diameter of the hollowfiber is preferably 20 μm to 400 μm, more preferably 30 μm to 300 μm,and still more preferably 40 μm to 200 μm.

In addition, in the case of the hollow fiber membrane, the percentage ofhollowness of the hollow fiber is preferably 15 to 55%, more preferably20 to 50%, and still more preferably 25 to 45%, in view of therelationship between the pressure loss of a fluid flowing through ahollow part and the buckling pressure.

A method for adjusting the outer diameter or the percentage ofhollowness of the hollow fiber in the hollow fiber membrane to fall inthe above-mentioned range is not particularly limited. For example, theadjustment can be made by appropriately changing the shape of adischarge hole of a spinneret for producing the hollow fiber or thedraft ratio which can be calculated by winding rate/discharge rate.

(1-6) Structure of Cross-Section

The separation membrane has, in the interior thereof, a plurality ofvoids each having a specified shape. As illustrated in FIG. 1, the voidsare observed in the cross-section of the membrane. The void as referredto herein has a structure in a cross-section, in which a depth (a sizein a membrane thickness direction, D₁) is 10 nm or more and 500 nm orless, a length (a size in a lengthwise direction of void, L₁) is 30 nmor more, and a ratio L₁/D₁ of the length to the depth is 2 or more.

The depth (D₁) of the void refers to a length in a depth direction ofthe void in the case where the membrane thickness direction of theseparation membrane is defined as the depth direction.

The length (L₁) of the void refers to a length in a lengthwise directionof the void in the case where a direction at which a distance when twopoints on an edge of the void are joined is the longest is defined asthe lengthwise direction of the void.

In addition, the term “cross-section of the membrane” as referred toherein indicates a cross-section parallel to a machine direction(lengthwise direction) during production and also parallel to athickness direction of the membrane, in the case of a flat membrane; andindicates a cross-section (longitudinal cross-section) parallel to athickness direction of the membrane and also parallel to a lengthwisedirection, in the case of a hollow fiber. In the case of press membraneformation or the like having no clear machine direction duringproduction, it refers to a cross-section in an arbitrary direction solong as it is parallel to a thickness direction of the membrane.Incidentally, the cross-section in the thickness direction of themembrane is a cross-section perpendicular to one face of the membrane.

As a result of extensive and intensive investigations made by thepresent inventors, it has been found that a membrane including voidshaving the above-described shape have a high water permeation amount.

As for reasons for this, first of all, when voids are present in theinterior of the membrane, a substantial thickness of the membranedecreases. Furthermore, when the voids have a depth (D₁) of 10 nm ormore and 500 nm or less, a length (L₁) of 30 nm or more, and a ratioL₁/D₁ of 2 or more, a contact area of the membrane with waterefficiently increases, and the membrane (namely, a cellulose ester, apolyamide or a polyester, or a mixture thereof, which are the maincomponent of the membrane) is easy to be swollen. It may be consideredthat the water permeation performance is improved due to these matters.In addition, when the depth (D₁) of the void is the above-describedupper limit value or less, excessive thinning, swelling, and unificationof voids can be suppressed, and there gives rise to an effect that anexcellent salt rejection is exhibited.

The depth (D₁) of the void is more preferably 20 nm or more and 400 nmor less, and still more preferably 30 nm or more and 300 nm or less. Thelength (L₁) of the void is more preferably 50 nm or more, and still morepreferably 100 nm or more. The ratio L₁/D₁ is more preferably 4 or more,and still more preferably 8 or more.

A width (W₁) of the void in a direction perpendicular to the depth andlengthwise directions is preferably 5 nm or more and 500 nm or less.When the width of the void falls within the above-described range,excessive connection of the voids is suppressed, and there gives rise toan effect that an excellent salt rejection is exhibited. The width ofthe void is more preferably 7 nm or more and 400 nm or less, and stillmore preferably 10 nm or more and 300 nm or less.

It is preferred that the lengthwise directions of the plurality of voidsare parallel to each other. The term “parallel” as referred to hereinrefers to the matter that in 50% or more of the number of voids observedin the cross-section observation as described later, an angle formed bythe lengthwise directions of the voids is within 30°. This angle is morepreferably within 20°, and still more preferably within 10°.

In addition, in the case where the separation membrane is a hollow fibermembrane, it is preferred that the lengthwise direction of the voidfollows the lengthwise direction of the separation membrane. When thelengthwise directions of the plurality of voids are parallel to eachother, even if the membrane is bent in the lengthwise direction of thevoid, the power is easy to be dispersed. Furthermore, when the voidsfollow the lengthwise direction of the membrane, the cellulose ester,the polyamide, or the polyester can take a structure continued in thelengthwise direction of the membrane, and a high membrane strength isobtained.

The term “the lengthwise direction of the void follows the lengthwisedirection of the separation membrane” refers to the matter that an angleformed by the lengthwise direction of the membrane and the lengthwisedirection of the void is within 20°. This angle is more preferablywithin 15°, and still more preferably within 10°.

Furthermore, a proportion of areas of voids per unit area in thecross-section in the above-described direction is described. The voidand other portion can be distinguished from each other by thecross-section observation. The proportion of the void area occupying inthe observation area (namely, a projected area of the cross-section),namely an occupancy rate {(S₁/S₁₀)×100} expressed by a projected area(observation area) S₁₀ of the measuring surface (cross-section) and anoccupation area S₁ of the voids is preferably 0.5% to 30%, morepreferably 1.0% to 20%, and especially preferably 2.0% to 15%. When thisproportion is 0.5% or more, an effect for increasing the waterpermeation amount due to substantial thinning is high; and when it is30% or less, a high separation performance is realized, becauseunification of the voids is suppressed.

A structure of cross-section of the separation membrane (internalstructure of the separation membrane) can be observed by AFM (atomicforce microscope). The observation area is an area of an observationvisual field and indicates an area of a region surrounded by an outeredge of the observation visual field.

A method for producing a membrane having the foregoing void structure isnot particularly limited. For example, there is exemplified a method inwhich a solvent-free resin composition prepared by heat melting of theraw materials is discharged from a slit-shaped spinneret, followed bycooling for solidification. In addition, there is also exemplified amethod in which a solution of a resin composition dissolved in a solventis cast on a glass plate or the like, followed by evaporating all thesolvent. Furthermore, there is also exemplified a method in which asolution of a resin composition dissolved in a solvent is dischargedfrom a slit-shaped spinneret and then evenly solidified in a thicknessdirection, and the solvent in the solution is evenly extracted in thethickness direction.

In these methods, the additive (C) is added, the mixture is subjected tomembrane formation without being made completely compatible with thecellulose ester, the polyamide or the polyester, or the mixture thereof,and the additive (C) is extracted, thereby obtaining voids and a groovestructure. Although a method for not making the additive (C) completelycompatible is not particularly limited, examples thereof include amethod of using a plasticizer which is low in compatibility with thecellulose ester, the polyamide or the polyester, or the mixture thereof;a method of melting the mixture at a low kneading strength and/or a lowtemperature; and the like.

In addition, the separation membrane is preferably homogeneous in astructure of cross-section in a thickness direction. The meanings of theterm “homogenous” are described in a second embodiment.

(1-7) Surface Structure

The present inventors have found that, as illustrated in FIG. 1, themembrane having a specified void structure in the interior thereof has aspecified groove structure on the membrane surface, and the groovestructure of the membrane surface reflects a void structure in theinterior of the membrane. It is preferred that the separation membraneof the present embodiment has a groove including a specified structureon at least one membrane surface. The groove as referred to hereinindicates a groove structure having a length (L₂) of 30 nm or more and awidth (W₂) of 5 nm or more and 500 nm or less. In addition, a ratioL₂/W₂ of the length to the width of the groove is preferably 2 or more.

The length (L₂) of the groove refers to a length in a lengthwisedirection of the groove in the case where a direction at which adistance when two point on an edge of the groove are joined is thelongest is defined as the lengthwise direction of the groove.

The width (W₂) of the groove refers to a length of the longest width inthe case where a direction perpendicular to the lengthwise direction ofthe groove and perpendicular to the thickness direction of the membraneis defined as a width direction.

As a result of extensive and intensive investigations made by thepresent inventors, it has been found that the membrane having a grooveincluding the above-described structure has a high water permeationamount. First of all, it may be considered that when the groove ispresent on the membrane surface, a contact area of the membrane withwater becomes large, and therefore, a degree of swelling of the membrane(namely, the cellulose ester, the polyamide or the polyester, or themixture thereof) increases. Furthermore, when the groove has a length of30 nm or more, a width of 5 nm or more, and a ratio L₂/W₂ of 2 or more,the membrane is more easily swollen. In addition, when the width of thegroove is the above-described upper limit value or less, excessiveswelling can be suppressed, and there gives rise to an effect that alarge lowering of the salt rejection can be suppressed.

The length of the groove is more preferably 40 nm or more, and stillmore preferably 50 nm or more. The width of the groove is morepreferably 7 nm or more and 400 nm or less, and still more preferably 10nm or more and 300 nm or less.

It is preferred that the lengthwise directions of the plurality ofgrooves are parallel to each other. The term “parallel” as referred toherein refers to the matter that in 50% or more of the number of groovesobserved in the cross-section observation as described later, an angleformed by the lengthwise directions of the grooves is within 30°. Thisangle is more preferably within 20°, and still more preferably within10°. In addition, in the case where the membrane is a hollow fibermembrane, it is preferred that the lengthwise direction of the groovefollows the lengthwise direction of the membrane.

When the lengthwise directions of the plurality of grooves are parallelto each other, even if the membrane is bent in the lengthwise directionof the groove, the power is easy to be dispersed, and the membrane ishardly broken. In particular, when the groove is parallel to thelengthwise direction of the membrane, such an effect is high. The term“the lengthwise direction of the groove follows the lengthwise directionof the separation membrane” refers to the matter that an angle formed bythe lengthwise direction of the membrane and the lengthwise direction ofthe groove is within 20°. This angle is more preferably within 15°, andstill more preferably within 10°.

Furthermore, a proportion of areas of grooves per unit area in thesurface of the membrane is described. The groove and other portion canbe distinguished from each other by the cross-section observation. Anoccupancy rate [(S₂/S₂₀)×100] of an occupation area (S₂) of the groovesin the measuring area, namely a projected area (S₂₀) of the surface ofthe membrane is preferably 0.5% to 20%, more preferably 1.0% to 15%, andespecially preferably 2.0% to 10%. When this occupancy rate is 0.5% ormore, an effect of increasing the water permeation amount is high,because the surface area becomes sufficiently large; and when it is 20%or less, a stable separation performance is realized, because thesurface structure of the membrane is stable.

Similar to the structure of cross-section of the separation membrane(internal structure of the separation membrane), the surface structureof the separation membrane can be observed by AFM (atomic forcemicroscope).

(2) Second Embodiment: Composite Separation Membrane

The membrane of the present embodiment is a composite separationmembrane including at least a first layer and a second layer.

(2-1) First Layer

As the first layer, the constitution of the separation membrane of thefirst embodiment as described above is adopted.

(2-2) Second Layer

The separation membrane of the present embodiment includes, in additionto the above-described first layer, at least the second layer. Thecomposite separation membrane is hereinafter sometimes referred tosimply as a composite membrane.

Examples of the composition and the structure of the second layer arehereunder described, but it should not be construed that the presentinvention is limited thereto.

(2-2-1) Main Component

It is preferred that the second layer includes, as a main componentthereof, at least one compound selected from the group consisting of thecellulose ester (A), the polyamide (E), and the polyester (F) asdescribed in the (1-1) section.

In the case of containing the above-described main component, the secondlayer is excellent in adhesion to the first layer. In particular, it ispreferred that the second layer has the same chemical composition as inthe first layer. For example, in the case where the first layerincludes, as the main component thereof, a cellulose ester, it ispreferred that the second layer also includes a cellulose ester as themain component thereof. In addition, it is preferred that the kind ofthe cellulose ester is identical between the first layer and the secondlayer. The same is also applicable to the polyester and the polyamide.In addition, in the case where the first layer includes, as the maincomponent thereof, a mixture of compounds of plural kinds, it ispreferred that the second layer also includes, as the main componentthereof, a mixture of compounds of the same kinds in the same ratio asin the first layer.

(2-2-2) Plasticizer

A complexing resin composition constituting the second layer of thecomposite separation membrane may also contain the plasticizer (B) asdescribed in (1-2) section.

(2-2-3) Additive

A complexing resin composition constituting the second layer of thecomposite separation membrane may also contain the additive (C) asdescribed in the (1-3) section. In the case of containing the additive(C), it becomes possible to improve the permeation performance,particularly when used as a membrane for water treatment,

Specific examples of the additive (C) are the same as those described inthe above-described (1-3) section.

After the complexing resin composition is heated and melted to form thesecond layer of the composite separation membrane, though the additive(C) may remain in the second layer, a part or the whole of the additive(C) may be eluted from the second layer into water. In the case wherethe additive (C) is eluted into water, traces formed by elution of theadditive (C) become voids in the membrane to improve the permeationperformance.

(2-2-4) Antioxidant

It is preferred that a complexing resin composition constituting thesecond layer of the composite separation membrane contains theantioxidant as described in the (1-4) section. Specific examples of theantioxidant are the same as those described in the above-described (1-4)section.

(2-3) Layer Structure of Composite Membrane

It is preferred that the first layer and the second layer in thecomposite membrane are each homogeneous in structure of a cross-sectionin a thickness direction. The term “homogeneous in structure of across-section” as used herein refers to a state in which no change instructure is observed, when the cross-section in the above-mentionedthickness direction of the membrane is continuously observed in thethickness direction from one surface side of the membrane to the othersurface side under a scanning electron microscope having a magnificationof 1,000 times. Here, strains in structure of the cross-section and thelike exerting an influence on the surface shape of the membrane are notregarded as changes in structure.

For example, a hollow fiber membrane obtained by discharging asolvent-free resin composition melted by heating from a spinneret andthereafter performing cooling and solidification, a membrane obtained bydischarging a solution in which a resin composition is dissolved in asolvent from a spinneret, thereafter evenly solidifying it in athickness direction and evenly extracting the solvent in the solution inthe thickness direction, and the like are a membrane homogeneous instructure of the cross-section, because the above-mentioned changes instructure are not confirmed.

On the other hand, when a solution in which a resin composition isdissolved in a solvent is discharged from a spinneret and thereafterunevenly solidified in a thickness direction, for example, when both orone surface is rapidly solidified and an inside is slowly solidified,extraction of the solvent in the solution is liable to become uneven inthe thickness direction. Therefore, the changes in structure areconfirmed in the thickness direction of the membrane, and a membranenon-homogeneous in structure of the cross-section is likely formed. Amembrane generally called an asymmetric membrane, which has a denseseparation functional layer partially in a thickness direction of themembrane by a non-solvent phase separation method, a heat-induced phaseseparation method or the like, is a membrane non-homogeneous instructure of the cross-section.

The composite separation membrane may be constituted of two layers ofthe first layer and the second layer, and so long as at least these twolayers are included, the composite separation membrane may also beconstituted of three or more layers including another layer.

In the case where the composite separation membrane is constituted oftwo layers of the first layer and the second layer, though any one ofthese layers may be an outer layer, it is preferred that the first layeris the outer layer. What the first layer is the outer layer is preferredfrom the standpoint that the surface structure as described in (1-7) isobtained.

In the case where the composite separation membrane is constituted ofthree or more layers, though the lamination order is not particularlylimited, it is preferred that the first layer is an outermost layer.What the first layer is the outermost layer is preferred from thestandpoint that the surface structure as described in (1-7) is obtained.

The “outer layer” and the “outermost layer” refer to a layer which isexposed on either one of the two surfaces of the membrane. Therefore, sofar as a hollow fiber membrane is concerned, the layer on any of thehollow part side or the opposite side thereto is called the “outerlayer” or the “outermost layer”.

In the case where the hollow fiber membrane has a plurality of layerseach having the same composition but having different occupancy rate ofvoids and rate of hole area or different rate of hole area, these layersare recognized as a separate layer from each other, and therefore, thishollow fiber membrane is corresponding to the constitution “includingthe first layer and the second layer”.

(2-4) Shape of Cross-Section of Composite Membrane

In the composite separation membrane, the first layer preferably has athickness of 0.01 μm to 90 μm. When the thickness of the first layer is0.01 μm or more, a good separation performance is obtained. In addition,when the thickness of the first layer is 90 μm or less, a goodpermeation performance is obtained.

The thickness of the first layer is more preferably 0.05 μm or more,still more preferably 0.1 μm or more, especially preferably 0.3 μm ormore, and most preferably 0.5 μm or more. In addition, the thickness ofthe first layer is more preferably 10 μm or less, still more preferably5 μm or less, especially preferably 2 μm or less, and most preferably 1μm or less.

Although the shape of the composite separation membrane is notparticularly limited, a hollow fiber membrane or a flat membrane ispreferably adopted. Of these, the hollow fiber membrane is morepreferred, because it is possible to be efficiently filled in a module,thereby being able to enlarge an effective membrane area per unit volumeof the module.

From the viewpoint of improving the permeation performance, thethickness of the whole of the composite separation membrane ispreferably 100 μm or less, more preferably 50 μm or less, and still morepreferably 30 μm or less. Meanwhile, from the viewpoint of obtaining asufficient membrane strength, the thickness of the whole of thecomposite separation membrane is preferably 2 μm or more, morepreferably 3 μm or more, and still more preferably 4 μm or more.

In the case of the hollow fiber membrane, from the viewpoint ofachieving both the effective membrane area at the time when filled inthe module and the membrane strength, an outer diameter of the hollowfiber is preferably 20 μm to 400 μm, more preferably 30 μm to 300 μm,and still more preferably 40 μm to 200 μm.

In addition, in the case of the hollow fiber membrane, the percentage ofhollowness of the hollow fiber is preferably 15 to 55%, more preferably20 to 50%, and still more preferably 25 to 45%, in view of therelationship between the pressure loss of a fluid flowing through ahollow part and the buckling pressure.

A method for adjusting the outer diameter or the percentage ofhollowness of the hollow fiber in the hollow fiber membrane to fall inthe above-mentioned range is not particularly limited. For example, theadjustment can be made by appropriately changing the shape of adischarge hole of a spinneret for producing the hollow fiber or thedraft ratio which can be calculated by winding rate/discharge rate.

(2-5) Rate of Hole Area of Composite Membrane

In the composite separation membrane, it is preferred that an occupancyrate V_(A) of voids in the cross-section of the first layer and a rateof hole area H_(B) of the second layer satisfy the following relationalformula.V _(A) <H _(B)

What this relational formula is satisfied is preferred from thestandpoint that it becomes possible to achieve both the permeationperformance and the separation performance. Measurement conditions ofthe occupancy rate of voids and rate of hole area are described indetail in Examples.

The occupancy rate V_(A) of voids in the first layer is preferably 0.5to 30%, more preferably 2.0 to 20%, and still more preferably 3.0 to15%. When the occupancy rate V_(A) of voids in the first layer isallowed to fall within the above-described range, it becomes possible toachieve both the permeation performance and the separation performance.

The rate of hole area H_(B) of the second layer is preferably 5 to 50%,more preferably 10 to 40%, and still more preferably 15 to 30%. When therate of hole area H_(B) of the second layer is allowed to fall withinthe above-described range, the permeation performance becomes good.

Although a method for adjusting the occupancy rate of voids and rate ofhole area in the first layer and the second layer is not particularlylimited, examples thereof include a method in which from the respectivelayers of the composite separation membrane obtained by membraneformation using the resin composition containing the plasticizer and/orthe additive of the preferred kind and amount as described above, theplasticizer and/or the additive is eluted into water, thereby formingvoids; and the like.

(2-6) Surface Structure of Composite Membrane

The membrane of the second embodiment can have the same surfacestructure as in the separation membrane of the first embodiment asdescribed above. Such a surface structure is also applied to the presentembodiment. In particular, in the case where the first layer ispositioned on the membrane surface, the same surface structure as in thefirst embodiment is obtained.

(3) Physical Properties of Separation Membrane

Physical properties of the separation membrane of each of theembodiments as described above are hereunder described.

(3-1) Membrane Permeation Flux

The separation membrane of the present invention preferably has amembrane permeation flux of 3.0 L/m²/day or more, in order to exhibit agood permeation performance, particularly when used as a water treatmentmembrane. Measurement conditions of the membrane permeation flux aredescribed in detail in Examples. The membrane permeation flux is morepreferably 4.0 L/m²/day or more, and still more preferably 9.0 L/m²/dayor more. The higher membrane permeation flux is preferred. However, anupper limit thereof is 300 L/m²/day in view of a balance with the saltrejection.

(3-2) Salt Rejection

The separation membrane preferably has a salt rejection of 30.0 to99.5%, in order to exhibit a good separation performance, particularlywhen used as a water treatment membrane. Measurement conditions of thesalt rejection are described in detail in Examples. The salt rejectionis more preferably 50.0 to 99.5%, and still more preferably 80.0 to99.5%.

(3-3) Tensile Elasticity

The separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.In particular, it is preferred that the tensile elasticity in alengthwise direction of the separation membrane falls within this range.The term “lengthwise direction” as used herein refers to a machinedirection during production. Measurement conditions of the tensileelasticity are described in detail in Examples.

When the tensile elasticity in the lengthwise direction is 1,000 MPa ormore, an appropriate strength is obtained. When the tensile elasticityin the lengthwise direction is 6,500 MPa or less, flexibility suitablefor incorporating the separation membrane into a membrane module isrealized. The tensile elasticity is preferably 1,500 to 6,000 MPa, morepreferably 1,800 to 5,500 MPa, and still more preferably 2,200 to 5,000MPa.

(3-4) Tensile Strength

The separation membrane preferably has a tensile strength of 80 MPa ormore, in order to exhibit a good membrane strength. Measurementconditions of the tensile strength are described in detail in Examples.The tensile strength is more preferably 90 MPa or more, and still morepreferably 100 MPa or more. The higher tensile strength is preferred.However, a practical upper limit thereof is 300 MPa.

(4) Additive

The separation membrane may contain an additive other than the additivesdescribed above, within a range not impairing the effect of the presentinvention. For example, an organic lubricant, a crystal nucleatingagent, organic particles, inorganic particles, a terminal blockingagent, a chain extender, an ultraviolet absorber, an infrared absorber,a coloration preventing agent, a delustering agent, an antimicrobialagent, an electrification suppressing agent, a deodorant, a flameretardant, a weather-resistant agent, an antistatic agent, anantioxidant, an ion-exchanging agent, an antifoaming agent, a colorpigment, a fluorescent whitening agent, a dye, and so on can be used.

(5) Type of Membrane

The separation membrane of the present invention is a membraneparticularly usable for water treatment. Specifically, examples of thewater treatment membrane include microfiltration membranes,ultrafiltration membranes, nanofiltration membranes, reverse osmosismembranes, forward osmosis membranes, and gas separation membranes. Theseparation membrane of the present invention is preferably appliedparticularly to the nanofiltration membranes, the reverse osmosismembranes, the forward osmosis membranes, and the gas separationmembranes.

2. Production Method of Separation Membrane

A method for producing the separation membrane of the present inventionis specifically described below, taking as an example the case where theseparation membrane is a hollow fiber membrane, but should not beconstrued as being limited thereto.

As the above-described production method of separation membrane, meltspinning is preferably applied.

The melt spinning is a formation method of membrane including a step ofmelting raw materials by heating to prepare a solvent-free resincomposition; and a step of subsequently discharging this resincomposition from a slit-shaped spinneret, followed by cooling forsolidification. The melt spinning is applicable to the production of anyof a flat membrane and a hollow fiber membrane.

Although the step of preparing a resin composition is not limited to aspecific method, a twin-screw extruder is preferably used. In addition,a twin-screw extruder including a screw having a flight region and akneading disk region may be used, and a twin-screw extruder including ascrew constituted of only a flight region may also be used. What thescrew is constituted of only a flight region is preferred from thestandpoint that the strength of kneading can be made low.

Examples of the raw materials of the separation membrane include theabove-described cellulose ester (A), polyamide (E), polyester (F),plasticizer (B), additive (C), and antioxidant (D). Specific examples ofthe respective raw materials are those as described above.

The content of the plasticizer (B) in the total amount of the rawmaterials (namely, the weight of the resin composition obtained bymelting) is preferably 1 to 26% by weight in the resin composition whichforms the first layer. When the content of the plasticizer (B) is 1% byweight or more, the thermoplasticity of the cellulose ester (A), thepolyamide (E) or the polyester (F), or the mixture thereof and thepermeation performance of the separation membrane become good. When thecontent of the plasticizer (B) is 26% by weight or less, the separationperformance and the membrane strength of the separation membrane becomegood. The content of the plasticizer (B) is more preferably 5 to 24% byweight, and still more preferably 14 to 22% by weight.

In addition, in the resin composition which forms the second layer, thecontent of the plasticizer (B) is preferably 10 to 50% by weight. Whenthe content thereof is 10% by weight or more, the thermoplasticity ofthe cellulose ester and the permeation performance of the compositeseparation membrane become good. When the content thereof is 50% byweight or less, the membrane strength of the composite separationmembrane becomes good. The content of the plasticizer is more preferably15 to 45% by weight, and still more preferably 20 to 40% by weight.

The content of the additive (C) at the time of melt spinning in thetotal amount of the raw materials is preferably 0.5 to 10% by weight inthe resin composition which forms the first layer. When the content ofthe additive (C) is 0.5% by weight or more, the permeation performanceof the separation membrane becomes good. When the content of theadditive (C) is 10% by weight or less, the separation performance andthe membrane strength of the separation membrane become good. Thecontent of the additive (C) is more preferably 1.0 to 8.0% by weight,and still more preferably 2.0 to 6.0% by weight.

In addition, in the resin composition which forms the second layer, thecontent of the additive (C) at the time of melt spinning in the totalamount of the raw materials is preferably 5 to 50% by weight. When thecontent thereof is 5% by weight or more, the permeation performance ofthe composite separation membrane becomes good. When the content thereofis 50% by weight or less, the membrane strength of the separationmembrane becomes good. The content of the additive (C) is morepreferably 8 to 45% by weight, and still more preferably 10 to 40% byweight.

The content of the antioxidant (D) in the total amount of the rawmaterials is preferably 0.005 to 0.500% by weight relative to thecomposition to be subjected to melt spinning.

On the occasion when the resin composition including, as a maincomponent thereof, the cellulose ester (A), the polyamide (E) or thepolyester (F) is formed into a hollow fiber by the melt spinning method,the spinning temperature (the temperature of the spinning pack) ispreferably (Tm+5° C.) to (Tm+50° C.), when the crystal meltingtemperature of the resin composition in temperature rise measurementwith a differential scanning calorimeter (DSC) is defined as Tm.Measurement conditions of DSC are described in detail in Examples.

The spinning temperature is more preferably (Tm+5° C.) to (Tm+40° C.),and still more preferably from (Tm+5° C.) to (Tm+30° C.). In the presentinvention, by suppressing this spinning temperature lower than usual,the separation performance of the separation membrane is more improved,and the membrane strength is more increased.

In producing the hollow fiber separation membrane, various spinneretscan be used. Specifically, a spinneret of a C-shaped slit, a spinnerethaving one discharge hole formed by arranging a plurality of (2 to 5)arcuate (arc-shaped) slit parts, a tube-in orifice type spinneret, andso on can be used.

For example, in the case of producing a membrane formed of a singlelayer as in the membrane of the first embodiment, a spinneret having onedischarge hole may be used.

In the case of producing a composite membrane, the resin compositions ofthe respective layers as melted by the above-described method aregathered within a spinneret having a multi-tube nozzle in which achannel of gas is arranged in the center thereof, and complexed. At thistime, the shape of a channel space within the spinneret for the resincomposition constituting each of the layers is properly altered inconformity with the melt viscosity of the resin composition and thedesired shape of the cross-section of the composite membrane to beproduced. In addition, the discharge amount of the resin compositionconstituting each of the layers from the spinneret is properly alteredby, for example, a rotation number of a gear pump, etc., in conformitywith the desired shape of the cross-section of the composite membrane tobe produced.

The thermoplastic resin composition melted is discharged downwards fromthe discharge hole of the spinneret which is assembled in a lower partof the spinning pack. Here, a distance H from a lower surface of thespinneret to an upper end of a cooling apparatus (chimney) is preferably0 mm to 50 mm, more preferably 0 mm to 40 mm, and still more preferably0 mm to 30 mm.

When the hollow fiber discharged from the spinneret is cooled, atemperature of the cooling air of the cooling apparatus (chimney) ispreferably 5 to 25° C. In addition, an air velocity of the cooling airis preferably 0.8 to 2.0 m/sec, more preferably 1.1 to 2.0 m/sec, andstill more preferably 1.4 to 2.0 m/sec.

The hollow fiber cooled with the cooling apparatus is wound by a winder.The draft ratio which can be calculated by a winding rate/discharge rateis preferably 200 to 1,000, more preferably 300 to 900, and still morepreferably 400 to 800.

The spun hollow fiber may be further drawn. A drawing method is notparticularly limited. For example, the temperature is elevated to atemperature at which drawing is performed, while transferring the hollowfiber membrane before drawing onto heat rolls, and the drawing can beperformed in a single stage or in multiple stages of two or more stages,utilizing a difference in peripheral speed among the heat rolls.

The range of the temperature of the hollow fiber membrane in the drawingstep is preferably 20 to 250° C., more preferably 20 to 220° C., andstill more preferably 20 to 200° C. A total draw ratio is preferably1.05 to 1.50, more preferably 1.10 to 1.45, and still more preferably1.15 to 1.40. In addition, heat setting may be performed during or afterdrawing as needed. A heat setting temperature is preferably 80 to 240°C.

Although the thus obtained hollow fiber membrane can be used as it is,in the case where the solubility of the additive (C) in water is low, itis preferred that the separation membrane is dipped in an alcohol or analcohol aqueous solution capable of dissolving the additive (C) therein,thereby eluting at least a part of the additive (C).

Before using the hollow fiber membrane, it is preferred that the surfaceof the membrane is hydrophilized with, for example, analcohol-containing aqueous solution, an alkali aqueous solution, etc.

3. Module

The separation membrane of the present invention may be incorporatedinto the separation membrane module when used. The separation membranemodule includes a membrane bundle constituted of a plurality of hollowfiber membranes and a case accommodating this membrane bundle therein.Either ends or one end of the membrane bundle is fixed within theabove-described case with polyurethane, an epoxy resin, or the like.

So far as a flat membrane is concerned, it is fixed to a support, or themembranes are stuck to each other to form an envelope-shaped membrane,and further installed to a water collection tube or the like as needed,thereby achieving modularization.

In a case for reverse osmosis membrane, a hole for feeding the mixedsolution, a hole through which purified water having permeated throughthe membrane passes, and a hole through which concentrated waste waterpasses are provided. In a case for pressure-retarded osmosis method orforward osmosis method, a cross-flow is employed, and two holes forsupplying the mixed solution and two holes through which water with avaried salt concentration passes are provided.

4. Others

The matters described in the present description can be combined witheach other. For example, the present invention can also be expressed asfollows.

(1) A separation membrane including, as a main component thereof, atleast one compound selected from the group consisting of a celluloseester, a polyamide, and a polyester,

in which the separation membrane has, in an interior thereof, aplurality of voids each having a depth (D₁) of 10 nm or more and 500 nmor less, a length (L₁) of 30 nm or more, and a ratio L₁/D₁ of the lengthto the depth in a range of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(2) The separation membrane according to (1), in which a lengthwisedirection of the void follows a lengthwise direction of the separationmembrane.

(3) The separation membrane according to (1) or (2), in which, when aprojected area of a cross-section of the separation membrane is definedas S₁₀, and an occupation area of the voids is defined as S₁, anoccupancy rate of the voids, expressed by {(S₁/S₁₀)×100}, is 0.5% ormore and 30% or less.

(4) The separation membrane according to any one of (1) to (3), having,on at least one surface thereof, a plurality of grooves each having alength (L₂) of 30 nm or more, a width (W₂) of 5 nm or more and 500 nm orless, and a ratio L₂/W₂ of the length to the width in a range of 2 ormore.

(5) A separation membrane including, as a main component thereof, atleast one compound selected from the group consisting of a celluloseester, a polyimide, and a polyester,

in which the separation membrane has, on at least one surface thereof, aplurality of grooves each having a length (L₂) of 30 nm or more, a width(W₂) of 5 nm or more and 500 nm or less, and a ratio L₂/W₂ of the lengthto the width in a range of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(6) The separation membrane according to (4) or (5), in which alengthwise direction of the groove follows a lengthwise direction of theseparation membrane.

(7) The separation membrane according to any one of (4) to (6), inwhich, when a projected area of a surface of the separation membrane isdefined as S₂₀, and an occupation area of the grooves is defined as S₂,an occupancy rate of the grooves in the surface, as expressed by{(S₂/S₂₀)×100}, is 0.5% or more and 20% or less.

(8) A separation membrane including at least a first layer and a secondlayer,

in which the first layer includes, as a main component thereof, at leastone compound selected from the group consisting of a cellulose ester, apolyamide, and a polyester,

the first layer has, in an interior thereof, a plurality of voids eachhaving a depth (D₁) of 10 nm or more and 500 nm or less, a length (L₁)of 30 nm or more, and a ratio L₁/D₁ of the length to the depth in arange of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(9) The separation membrane according to (8), in which a lengthwisedirection of the void follows a lengthwise direction of the separationmembrane.

(10) The separation membrane according to (8) or (9), in which, when aprojected area of a cross-section of the separation membrane is definedas S₁₀, and an occupation area of the voids is defined as S₁, anoccupancy rate of the voids in the cross-section of the separationmembrane, as expressed by {(S₁/S₁₀)×100}, is 0.5% or more and 30% orless.

(11) The separation membrane according to any one of (8) to (10),having, on at least one surface thereof, a plurality of grooves eachhaving a length (L₂) of 30 nm or more, a width (W₂) of 5 nm or more and500 nm or less, and a ratio L₂/W₂ of the length to the width in a rangeof 2 or more.

(12) A separation membrane including at least a first layer and a secondlayer,

in which the first layer includes, as a main component thereof, at leastone compound selected from the group consisting of a cellulose ester, apolyamide, and a polyester,

the separation membrane has, on at least one surface thereof, aplurality of grooves each having a length (L₂) of 30 nm or more, a width(W₂) of 5 nm or more and 500 nm or less, and a ratio L₂/W₂ of the lengthto the width in a range of 2 or more, and

the separation membrane has a tensile elasticity of 1,000 to 6,500 MPa.

(13) The separation membrane according to (11) or (12), in which alengthwise direction of the groove follows a lengthwise direction of theseparation membrane.

(14) The separation membrane according to any one of (11) to (13), inwhich, when a projected area of a surface of the separation membrane isdefined as S₂₀, and an occupation area of the grooves is defined as S₂,an occupancy rate of the grooves in the surface, as expressed by{(S₂/S₂₀)×100}, is 0.5% or more and 20% or less.

(15) The separation membrane according to any one of (8) to (14), inwhich an occupancy rate V_(A) of voids in a cross-section of the firstlayer and a rate of hole area H_(B) of the second layer satisfy arelation: V_(A)<H_(B).

(16) The separation membrane according to any one of (8) to (15), inwhich the first layer has a thickness of 0.01 μm to 90 μm.

(17) The separation membrane according to any one of (1) to (16), inwhich the separation membrane has a shape of a hollow fiber.

(18) The separation membrane according to (17), in which the hollowfiber has an outer diameter of 20 μm to 400 μm.

(19) The separation membrane according to any one of (1) to (18), inwhich the separation membrane includes, as the main component thereof,the cellulose ester, and

the separation membrane includes, as the cellulose ester, at least oneof cellulose acetate propionate and cellulose acetate butyrate.

(20) The separation membrane according to any one of (1) to (19), inwhich the separation membrane includes, as the main component thereof,the polyamide, and

the separation membrane includes, as the polyamide, at least one ofnylon 6 and nylon 66.

(21) The separation membrane according to any one of (1) to (20), inwhich the separation membrane includes, as the main component thereof,the polyester, and

the separation membrane includes, as the polyester, a copolymer ofpolyethylene terephthalate with 5-sodium sulfoisophthalic acid.

(22) The separation membrane according to any one of (1) to (21), inwhich the separation membrane is at least one selected from the groupconsisting of a nanofiltration membrane, a reverse osmosis membrane, aforward osmosis membrane, and a gas separation membrane.

(23) A membrane module including the separation membrane according toany one of (1) to (22).

EXAMPLES

The present invention is more specifically described below showingExamples. However, the present invention should not be construed asbeing restricted thereby in any way.

The respective characteristic values in Examples were determined by thefollowing methods. In the following (3) to (7), (10) and (11),measurement and evaluation were performed in a state where eachseparation membrane was dried in vacuum at 25° C. for 8 hours.

(1) Average Degrees of Substitution for Cellulose Ester (A)

A method for calculating the average degrees of substitution for acellulose ester (A) in which acetyl groups and acyl groups are bonded tocellulose is as follows.

A cellulose ester (0.9 g) dried at 80° C. for 8 hours was weighed, anddissolved by adding 35 mL of acetone and 15 mL of dimethyl sulfoxide.Thereafter, 50 mL of acetone was further added thereto. With stirring,30 mL of a 0.5 N aqueous solution of sodium hydroxide was added,followed by saponification for 2 hours. Then, 50 mL of hot water wasadded, and a side surface of a flask was washed. Thereafter, titrationwas performed with 0.5 N sulfuric acid using phenolphthalein as anindicator. Separately, a blank test was performed by the same method asfor the sample. After the completion of the titration, a supernatant ofthe solution was diluted to 100 times, and the compositions of organicacids were measured using an ion chromatograph. From the measurementresults and the results of acid composition analysis with the ionchromatograph, the degrees of substitution were calculated by thefollowing formulae.TA=(B−A)×F/(1000×W)DSace=(162.14×TA)/[{1−(Mwace−(16.00+1.01))×TA}+{1−(Mwacy−(16.00+1.01))×TA}×(Acy/Ace)]DSacy=DSace×(Acy/Ace)

TA: Total organic acid amount (mL)

A: Sample titration amount (mL)

B: Blank test titration amount (mL)

F: Titer of sulfuric acid

W: Sample weight (g)

DSace: Average degree of substitution of acetyl groups

DSacy: Average degree of substitution of acyl groups

Mwace: Molecular weight of acetic acid

Mwacy: Molecular weight of another organic acid

Acy/Ace: Molar ratio of acetic acid (Ace) and another organic acid (Acy)

162.14: Molecular weight of a repeating unit of cellulose

16.00: Atomic weight of oxygen

1.01: Atomic weight of hydrogen

(2) Weight Average Molecular Weights (Mw) of Cellulose Ester (A),Polyamide (E), and Polyester (F)

A cellulose ester (A), a polyamide (E), or a polyester (F) wascompletely dissolved in tetrahydrofuran or N-methylpyrrolidone(hereinafter sometimes referred to as NMP) to a concentration of 0.15%by weight to prepare a sample for GPC measurement. Using this sample,GPC measurement was performed with Waters 2690 under the followingconditions to determine the weight average molecular weight (Mw) interms of polystyrene.

Column: Two TSK gel GMHHR-H columns (manufactured by Tosoh Corporation)were connected to each other.

Detector: Waters 2410, differential refractometer R1

Solvent for mobile phase: Tetrahydrofuran, NMP, or hexafluoroisopropanol

Flow rate: 1.0 mL/min

Injection amount: 200 μL

(3) Outer Diameter (μm) of Hollow Fiber

Cross-sections in a direction perpendicular to a lengthwise direction ofa hollow fiber (in a fiber diameter direction) and in a thicknessdirection of the membrane were observed and photographed by an opticalmicroscope, and the outer diameter (μm) of the hollow fiber wascalculated. The outer diameter of the hollow fiber was calculated using10 hollow fibers, and the average value thereof was obtained.

(4) Percentage of Hollowness of Hollow Fiber

A cross-section perpendicular to a lengthwise direction of a hollowfiber was observed and photographed by an optical microscope, and atotal area Sa of the cross-section and an area Sb of the hollow partwere measured. The percentage of hollowness was calculated using thefollowing formula. The percentage of hollowness was calculated using 10hollow fibers, and an average value thereof was obtained.Percentage of hollowness (%)=(Sb/Sa)×100

(5) Measurement of Void Shape

A hollow fiber membrane was cut with a microtome to obtain across-section parallel to a lengthwise direction and a diameterdirection of the hollow fiber membrane.

After cutting, the hollow fiber membrane was fixed onto a sample table,and the membrane cross-section was observed with an AFM, NanoScope VDimension Icon, manufactured by Bruker AXS. Before cross-sectionexposing, embedding with a resin was performed as needed. Afterperforming the tilt correction of the resulting image, the shape of thevoid was analyzed. This operation was performed in 10 regions (namely,visual fields), the shape of the void was measured, the occupation areaof the voids was calculated, and the lengthwise direction of the voidwas specified. Specific measurement conditions are as follows.

-   -   Scanning mode: Tapping mode (in air)    -   Probe: Silicon cantilever (manufactured by Bruker AXS)    -   Scanning range: 2 μm×2 μm    -   Scanning rate: 0.3 to 0.5 Hz    -   Number of pixels: 128×128 pixels or more    -   Measurement conditions: Room temperature in air

As described above, in the longitudinal cross-section of the hollowfiber, arbitrary 10 regions of different 2 μm-square ranges were chosen.As for all voids contained in a certain region, a direction at whichwhen two points on an edge of each void were joined is the longest indistance was defined as the lengthwise direction of the void, and thisdistance was measured as a length. However, in determining thelengthwise direction and measuring the length, among line segmentsjoining the two points on the edge of the void, one intersecting theedge of the void was excluded. In the case where the membrane thicknessor the thickness of the layer is less than 2 μm, the region is properlychanged from the 2 μm-square range such that one side of the observationrange becomes not more than the membrane thickness or the thickness ofthe layer.

The membrane thickness direction was defined as a depth direction. Inmeasuring the void shape, using cross-sectional analysis, a depth (D₁)of the void was measured, and extraction of the void corresponding tothe range of the above-described void shape was performed. Furthermore,such a void was measured for a length (L₁) and a width (W₁).

In determining the lengthwise direction of the void and measuring thelength of the void, in the case where the void was divided by an outeredge of the visual field, the measurement was performed considering theouter edge of the visual field as an outer edge of the void. However,the case where the outer edge diving the void was at an angle of morethan 15° against the lengthwise direction of the void to be divided wasexcluded from the measurement object of the void shape.

From the resulting void shape, a ratio L₁/D₁ of a length (L₁) in thelengthwise direction to a depth (D₁) of the void was determined. Voidshaving a ratio L₁/D₁ of 2 or more were extracted, and an average valueof each of L₁, D₁, and W₁ was calculated and classified as shown inTable 1.

Furthermore, in an AFM two-dimensional image, using an observation rangearea (projected area of the measuring surface) (S₁₀) and an occupationarea (S₁) of the voids, an occupancy rate of voids was calculatedaccording to the formula: [(S₁/S₁₀)×100]. In the following Examples andComparative Examples, since the 2 μm-square visual field was observed,the area S₁₀ was 4 μm².

(6) Measurement of Groove Shape

A hollow fiber was fixed onto a sample table with a pressure sensitiveadhesive double coated tape, and the membrane surface was observed withDimension FastScan manufactured by Bruker AXS in the same manner as inthe cross-sectional observation. After performing the tilt correction ofthe resulting image, the shape of the groove was analyzed. Thisoperation was performed in 10 visual fields, the shape of the groove wasmeasured, the occupation area of the grooves was calculated, and thelengthwise direction of the groove was specified. Specific measurementconditions are as follows.

-   -   Scanning mode: Nanomechanical mapping in air    -   Probe: Silicon cantilever (ScanAsyst-Air, manufactured by Bruker        AXS)    -   Maximum load: 10 nN    -   Scanning range: 2 μm×2 μm    -   Scanning rate: 1.0 Hz    -   Number of pixels: 128×128 pixels or more    -   Measurement conditions: In air

The surface was observed by the above-described method, and arbitrary 10regions of different 2 μm-square ranges were chosen. As for a pluralityof grooves contained in a certain region, a direction at which when twopoints on an edge of each groove were joined is the longest in distancewas defined as the lengthwise direction of the groove. However, amonglines joining the two points on the edge of the groove, one intersectingthe edge of the groove was excluded, thereby determining the direction.The direction perpendicular to the lengthwise direction of the groovewas defined as a width direction. In measuring the groove shape, usingcross-sectional analysis, a longest width (W₂) of the groove wasmeasured, and extraction of the groove corresponding to the range of theabove-described groove shape was performed. Furthermore, such a groovewas measured for a length (L₂) in the lengthwise direction and a depth(D₂).

In determining the lengthwise direction of the groove and measuring thelength of the groove, in the case where the groove was divided by anouter edge of the visual field, the measurement was performedconsidering the outer edge of the visual field as an outer edge of thegroove. However, the case where the outer edge diving the groove was atan angle of more than 15° against the lengthwise direction of the grooveto be divided was excluded from the measurement object of the grooveshape.

From the resulting groove shape, a ratio L₂/W₂ of a length (L₂) in thelengthwise direction to a longest width (W₂) of each groove wasdetermined. Voids having a ratio L₂/W₂ of 2 or more were extracted, andan average value of each of L₂, D₂, and W₂ was calculated and classifiedas shown in Table 1. Furthermore, in an AFM two-dimensional image, usingan observation range area (projected area of the measuring surface)(S₂₀) and an occupation area (S₂) of the grooves, an occupancy rate ofthe voids was calculated according to the formula: [(S₂/S₂₀)×100]. Inthe following Examples and Comparative Examples, since the 2 μm-squarevisual field was observed, the area S₂₀ was 4 μm².

(7) Crystal Melting Temperature (° C.) of Resin Composition for MeltSpinning

Using a differential scanning calorimeter DSC-6200, manufactured bySeiko Instruments Inc., about 5 mg of a resin composition sample driedin vacuum at 25° C. for 8 hours was set in an aluminum tray, increasedin temperature from −50° C. to 350° C. at a temperature rising rate of20° C./min, and thereafter held in a molten state for 5 minutes whilekeeping 350° C. A crystal melting peak observed at this time was takenas the crystal melting temperature (° C.). When a plurality of crystalmelting peaks appeared, the crystal melting peak which appeared on thehighest temperature side was employed.

(8) Permeation Performance (Membrane Permeation Flux (L/m²/day))

An aqueous solution of sodium chloride adjusted to a concentration of500 ppm, a temperature of 25° C., and a pH of 6.5 was fed at anoperation pressure of 0.75 MPa to a separation membrane, therebyperforming membrane filtration treatment. Based on the amount of theresultant permeate, the membrane permeation flux was determined by thefollowing formula:Membrane permeation flux (L/m²/day)=(Amount of permeate perday)/(Membrane area)

(9) Separation Performance [Salt Rejection (%)]

The membrane filtration treatment was performed under the sameconditions as in the case of the membrane permeation flux, and a saltconcentration of the resulting permeate was measured. From the saltconcentration of the resulting permeate and the salt concentration offeed water, a salt rejection was determined based on the followingformula. The salt concentration of permeate was determined from themeasured value of the electroconductivity.Salt rejection (%)=100×[1−{(Sodium chloride concentration inpermeate)/(Sodium chloride concentration in feed water)}]

When the separation membrane was a hollow fiber membrane in theabove-described (8) and (9), a small-sized module was produced asdescribed below, and the membrane filtration treatment was performed.

The hollow fiber membranes were bundled and inserted into apolycarbonate-made pipe, and thereafter, a thermosetting resin wasinjected into the pipe ends and cured to seal the ends of the hollowfiber membranes. The thermosetting resin having sealed the hollow fibermembranes was cut in a cross-section direction perpendicular to along-axis direction of the hollow fiber membranes to obtain openingsurfaces of the hollow fiber membranes, thereby preparing a small-sizedmodule for evaluation having a membrane area on an outer diameter basisof about 0.1 m².

(10) Tensile Elasticity (MPa)

A tensile elasticity (MPa) was measured in an environment of atemperature of 20° C. and a humidity of 65%, using a tensile tester(Tensilon UCT-100, manufactured by Orientec Co., Ltd.) under conditionsof a sample length of 100 mm and a tension rate of 100 mm/min. Themeasurement was repeated 5 times, and an average value thereof wasdefined as a tensile elasticity.

(11) Membrane Strength [Tensile Strength (MPa)]

A tensile strength (breaking strength) (MPa) was measured in anenvironment of a temperature of 20° C. and a humidity of 65%, using atensile tester (Tensilon UCT-100, manufactured by Orientec Co., Ltd.)under conditions of a sample length of 100 mm and a tension rate of 100mm/min. The measurement was repeated 5 times, and an average valuethereof was defined as a tensile strength.

(12) Rate of Hole Area H_(B) (%)

Using a scanning electron microscope, a cross-section of the secondlayer was observed and photographed at a magnification of 30,000 times,a transparent film or sheet was superimposed on the resultingcross-sectional photograph, and portions corresponding to fine poreswere painted over with an oil-based ink or the like. Subsequently, usingan image analyzer, an area of the foregoing region was determined. Thismeasurement was performed with respect to arbitrary 30 fine pores andaveraged to calculate an average pore area (m²). Subsequently, thenumber of fine pores per 3-μm square in the photograph in which theaverage pore diameter was calculated was counted and expressed in termsof the number of fine pores per 1 m², thereby calculating a fine poredensity (per m²). A rate of hole area was determined from the determinedaverage pore diameter and fine pore density according to the followingformula. Here, in calculating the rate of hole area, fine pores having afine pore diameter (minor axis in the case of an oval shape or a rodshape) of 1 nm or more were observed, and a pore area and a fine poredensity thereof were adopted.Rate of hole area (%)=(Average pore area)×(Fine pore density)×100[Cellulose Ester (A)]

Cellulose Ester (A1)

To 100 parts by weight of cellulose (cotton linter), 240 parts by weightof acetic acid and 67 parts by weight of propionic acid were added,followed by mixing at 50° C. for 30 minutes. After the mixture wascooled to room temperature, 172 parts by weight of acetic anhydridecooled in an ice bath and 168 parts by weight of propionic anhydridewere added as esterifying agents, and 4 parts by weight of sulfuric acidwas added as an esterifying catalyst, followed by stirring for 150minutes to conduct an esterification reaction. When the temperatureexceeded 40° C. in the esterification reaction, cooling was performed ina water bath. After the reaction, a mixed solution of 100 parts byweight of acetic acid and 33 parts by weight of water was added theretoas a reaction terminator for 20 minutes to hydrolyze excessiveanhydrides. Thereafter, 333 parts by weight of acetic acid and 100 partsby weight of water were added, followed by heating and stirring at 80°C. for 1 hour. After the completion of the reaction, an aqueous solutioncontaining 6 parts by weight of sodium carbonate was added. Celluloseacetate propionate precipitated was separated by filtration,subsequently washed with water, and thereafter dried at 60° C. for 4hours, thereby obtaining a cellulose ester (A1) (cellulose acetatepropionate). The average degrees of substitution of acetyl groups andpropionyl groups of cellulose acetate propionate obtained were 1.9 and0.7, respectively, and the weight average molecular weight (Mw) thereofwas 178,000.

Cellulose Ester (A2)

In 500 ml of deionized water, 50 g of cellulose (dissolving pulpmanufactured by Nippon Paper Industries Co., Ltd.) was immersed, andallowed to stand for 10 minutes. This was separated by a glass filter,drained, dispersed in 700 mL of acetic acid, sometimes mixed by shaking,and allowed to stand for 10 minutes. Subsequently, the same operationwas repeated again using new acetic acid. In a flask, 900 g of aceticacid and 0.9 g of concentrated sulfuric acid were put, and stirred.Thereto, 180 g of acetic anhydride was added, followed by stirring for60 minutes while cooling in a water bath so that the temperature did notexceed 40° C. After the completion of the reaction, an aqueous solutioncontaining 2 g of sodium carbonate was added. The cellulose esterprecipitated was separated by filtration, subsequently washed withwater, and thereafter dried at 60° C. for 4 hour, thereby obtaining acellulose ester (A2) (cellulose acetate). The amount of celluloseacetate obtained was 85.3 g, and the average degree of substitution ofcellulose acetate was 2.9.

[Polyamide (E)]

(E1)

Nylon 6 (Nylon 6 resin “Amilan”, manufactured by Toray Industries, Inc.)

(E2)

Nylon 66 (Nylon 66 resin “Amilan”, manufactured by Toray Industries,Inc.)

[Polyester (F)]

(F1)

Copolymer of polyethylene terephthalate with 5-sodium sulfoisophthalicacid

Magnesium acetate in an amount corresponding to 60 ppm as expressed interms of a magnesium atom relative to the polymer to be obtained, 58.1parts by weight of dimethyl terephthalate, 33.8 parts by weight ofethylene glycol, and 8.1 parts by weight of 5-sodium sulfoisophthalicacid dimethyl ester (manufactured by Sanyo Chemical Industries, Ltd.)were melted at 150° C. in a nitrogen atmosphere. Thereafter, thetemperature was raised to 230° C. over 3 hours while stirring; methanolwas distilled off; and the resultant was subjected to ester interchangereaction to obtain a condensation precursor.

About 100 kg of the condensation precursor was transferred into apolycondensation tank. Thereafter, antimony trioxide in an amountcorresponding to 250 ppm as expressed in terms of an antimony atom andtrimethyl phosphate in an amount corresponding to 50 ppm as expressed interms of a phosphorus atom, relative to the polymer to be obtained, werepreviously mixed in ethylene glycol in another mixing tank 30 minutesbefore the addition, and the mixture was stirred at normal temperaturefor 30 minutes. Thereafter, the resulting mixture was added to thecondensation precursor in the polycondensation tank.

Furthermore, five minutes after the addition, an ethylene glycol slurryof titanium oxide particles in an amount corresponding to 0.1% by weightas expressed in terms of a titanium oxide particle relative to thepolymer to be obtained was added. Furthermore, five minutes after theaddition, the reaction system was reduced in pressure, therebycommencing the reaction.

The interior of the reactor was subjected to temperature rise from 250°C. to 280° C. gradually, and simultaneously, the pressure was reduced to110 Pa. Times for reaching a final temperature and a final pressure wereeach set to 60 minutes.

Three hours after commencement of the pressure reduction, the reactionsystem was purged with nitrogen to return to normal pressure, therebyterminating the polycondensation reaction. The resultant was dischargedin a gut shape and cooled, followed by cutting to obtain pellets of thepolymer.

[Plasticizer (B)]

Plasticizer (B1)

Polyethylene glycol, weight average molecular weight: 600

Plasticizer (B2)

Polyvinylpyrrolidone (K30) (manufactured by BASF SE)

Plasticizer (B3)

Polyethylene glycol, weight average molecular weight: 1,000

[Additive (C)]

Additive (C1)

Glycerin

Additive (C2)

Diglycerin

Additive (C3)

Diglycerin oleate

Additive (C4)

Octyl phthalate

Additive (C5)

Dioctyl adipate

Additive (C6)

Polyethylene glycol, number average molecular weight: 8,300

Additive (C7)

Polyethylene glycol, number average molecular weight: 100,000

Additive (C8)

Polyethylene glycol, number average molecular weight: 300,000

Antioxidant (D1)

Bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite

Antioxidant (D2)

Hindered phenol-based antioxidant (Irganox (a registered trademark) 1098(manufactured by BASF SE))

Antioxidant (D3)

Hindered phenol-based antioxidant (Irganox (a registered trademark) 1010(manufactured by BASF SE))

Polyamide (E1)

Nylon 6 (Nylon 6 resin “Amilan”, manufactured by Toray Industries, Inc.)

Polyester (F1)

Copolymer of polyethylene terephthalate with 5-sodium sulfoisophthalicacid

Magnesium acetate in an amount corresponding to 60 ppm as expressed interms of a magnesium atom relative to the polymer to be obtained, 58.1parts by weight of dimethyl terephthalate, 33.8 parts by weight ofethylene glycol, and 8.1 parts by weight of 5-sodium sulfoisophthalicacid dimethyl ester (manufactured by Sanyo Chemical Industries, Ltd.)were melted at 150° C. in a nitrogen atmosphere. Thereafter, thetemperature was raised to 230° C. over 3 hours while stirring; methanolwas distilled off; and the resultant was subjected to ester interchangereaction to obtain a condensation precursor.

About 100 kg of the condensation precursor was transferred into apolycondensation tank. Thereafter, antimony trioxide in an amountcorresponding to 250 ppm as expressed in terms of an antimony atom andtrimethyl phosphate in an amount corresponding to 50 ppm as expressed interms of a phosphorus atom, relative to the polymer to be obtained, werepreviously mixed in ethylene glycol in another mixing tank 30 minutesbefore the addition, and the mixture was stirred at normal temperaturefor 30 minutes. Thereafter, the resulting mixture was added to thecondensation precursor in the polycondensation tank.

Furthermore, five minutes after the addition, an ethylene glycol slurryof titanium oxide particles in an amount corresponding to 0.1% by weightas expressed in terms of a titanium oxide particle relative to thepolymer to be obtained was added. Furthermore, five minutes after theaddition, the reaction system was reduced in pressure, therebycommencing the reaction. The interior of the reactor was subjected totemperature rise from 250° C. to 280° C. gradually, and simultaneously,the pressure was reduced to 110 Pa. Times for reaching a finaltemperature and a final pressure were each set to 60 minutes. Threehours after commencement of the pressure reduction, the reaction systemwas purged with nitrogen to return to normal pressure, therebyterminating the polycondensation reaction. The resultant was dischargedin a gut shape and cooled, followed by cutting to obtain pellets of thepolymer.

Production of Separation Membrane Example 1

82% by weight of cellulose ester (A1), 15.9% by weight of polyethyleneglycol (B1) having a weight average molecular weight of 600(manufactured by Sanyo Chemical Industries, Ltd.) as the plasticizer(B), 2.0% by weight of glycerin (C1) (manufactured by Wako Pure ChemicalIndustries, Ltd.), and 0.1% by weight ofbis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite (D1) asthe antioxidant (D) were melt-kneaded in a twin-screw extruder at 220°C. Thereafter, the resultant was introduced into a melt spinning packadjusted to 235° C. in spinning temperature, and spun downwards underconditions of a discharge rate of 60 g/min from a spinneret having 72spinneret holes (a type of forming one discharge hole by arranging 3arcuate slit parts, discharge hole radius: 0.60 mm, pitch between slits:0.10 mm, slit width: 0.08 mm). The spun hollow fibers were introducedinto a cooling apparatus (chimney) so that a distance H from a lowersurface of the spinneret to an upper end of the cooling apparatus was 30mm, cooled by cooling air at 25° C. and an air velocity of 1.5 m/sec,subjected to application of an oiling agent, bundled, and then wound bya winder at a draft ratio of 200.

As the twin-screw extruder as referred to in the Examples andComparative

Examples, one in which the screw was formed of only a flight region wasused. The separation membrane was immersed in a 50% ethanol aqueoussolution, thereby eluting at least a part of the additive (C).Thereafter, the resulting separation membrane was immersed in a 10 wt %aqueous solution of isopropyl alcohol for 1 hour, thereby performinghydrophilization.

The physical properties of the thus obtained separation membrane areshown in Table 2.

Examples 2 to 14 and Comparative Examples 1 and 2

Separation membranes were obtained in the same manner as in Example 1,except that the composition of the resin composition for melt spinningand the production conditions were each changed as shown in Tables 2 and4. The physical properties of the resulting separation membranes areshown in Tables 2 and 4. The resin compositions in Tables 2 and 4 areshown in Table 7.

Examples 15 to 32 and Comparative Examples 3 to 7

The raw materials were melt-kneaded in a twin-screw extruder at 220° C.so as to have compositions described in Tables 3 and 4 and thenpelletized to obtain resin compositions for melt spinning. The pelletswere dried in vacuum at 80° C. for 8 hours.

The dried pellets were fed into a twin-screw extruder and melted at 235°C. The resultant was introduced into a melt spinning pack adjusted to235° C. in spinning temperature as shown in the tables and spun. Usingthe same spinneret holes as in Example 1, spinning was performed in thesame manner as in Example 1, except that the spinning conditions asshown in Tables 3 and 4 were adopted.

Comparative Example 8

41% by weight of cellulose acetate (LT35), manufactured by DaicelCorporation, 49.9% by weight of N-methyl-2-pyrrolidone, 8.8% by weightof ethylene glycol, and 0.3% by weight of benzoic acid were dissolved at180° C. The resulting solution was defoamed under reduced pressure, thenspun downwards from spinneret holes (a type of forming one dischargehole by arranging 3 arcuate slit parts) at 160° C., and solidified in abath of 12° C. containing N-methyl-2-pyrrolidone/ethyleneglycol/water=4.25% by weight/0.75% by weight/95% by weight, after anelapse of a time of exposure to air of 0.03 seconds, followed by washingin water. Thereafter, heat treatment was performed in water of 60° C.for 40 minutes to obtain a hollow fiber membrane having an outerdiameter of 167 μm and a percentage of hollowness of 25%.

The resulting hollow fiber membrane had a membrane permeation flux of 87L/m²/day, a salt rejection of 97.2%, a tensile elasticity of 1,435 MPa,and a tensile strength of 72 MPa.

In the separation membranes of Examples 1 to 32, a plurality of voidswere present in the interior of each membrane. In addition, thelengthwise directions of these plural voids were parallel to each otherand followed the lengthwise direction of the hollow fiber membrane. Aplurality of grooves were present on the outer surface of the membrane.The lengthwise directions of these plural grooves were parallel to eachother and followed the lengthwise direction of the hollow fibermembrane.

The thus obtained membranes of Examples 1 to 32 had a membranepermeation flux of 3.0 L/m²/day or more, so that a good permeationperformance could be exhibited. Furthermore, the elasticity was 1,000MPa or more, so that a good membrane strength could be exhibited. Inconsequence, it was noted that the separation membranes of Examples 1 to32 have a high membrane strength and a high permeation performance.

The separation membranes of Comparative Examples 1, 4, 5, and 6 wereinferior in the permeation performance to the separation membranes ofExamples 1 to 32. Comparative Examples 5 and 7 were low in the saltrejection. In Comparative Examples 2, 7, and 8, the elasticity was lessthan 1,000 MPa.

Meanwhile, in Comparative Example 3, flowability was poor because ofexcessively high melt viscosity, so that thinning of spun yarns did notoccur, resulting in a failure of winding due to yarn breakage. Inconsequence, it was noted that, in the separation membranes of theComparative Examples which did not have configurations of the presentinvention, good separation performance and permeation performance, ormembrane strength cannot be exhibited.

Example 33

89.5 parts by weight of the nylon 6 resin (E1), 5 parts by weight ofpolyvinylpyrrolidone (K30) (B2), 5 parts by weight of polyethyleneglycol (Mw: 100,000) (C7), and 0.5 parts by weight of Irganox 1098 (D2)were melt-kneaded in a twin-screw extruder at 270° C. and thenpelletized to obtain a resin composition for melt spinning. The pelletswere dried in vacuum at 80° C. for 15 hours.

The dried pellets were fed into a twin-screw extruder and melted at 240°C. Thereafter, the resultant was introduced into a melt spinning packadjusted to 250° C. in spinning temperature, and spun downwards underconditions of a discharge rate of 60 g/min from a spinneret having 72spinneret holes (a type of forming one discharge hole by arranging 3arcuate slit parts, discharge hole radius: 0.60 mm, pitch between slits:0.10 mm, slit width: 0.08 mm). The spun hollow fibers were introducedinto a cooling apparatus (chimney) so that a distance H from a lowersurface of the spinneret to an upper end of the cooling apparatus was 30mm, cooled by cooling air at 25° C. and an air velocity of 1.5 m/sec,subjected to application of an oiling agent, bundled, and then wound bya winder at a draft ratio of 400. The physical properties of the thusobtained separation membrane are shown in Table 5.

A module including this separation membrane was prepared and immersed ina 10 wt % aqueous solution of isopropyl alcohol for 1 hour, therebyperforming hydrophilization. Thereafter, the membrane permeation fluxand separation performance were evaluated. The results are shown inTable 5. The resin composition in Table 5 is shown in Table 7.

Examples 34 to 38

Separation membranes were obtained in the same manner as in Example 33,except that the composition of the resin composition for melt spinningand the production conditions were each changed as shown in Table 4. Thephysical properties of the resulting separation membranes and theperformances of the separation membrane modules are shown in Table 5.The resin compositions in Table 5 are shown in Table 7.

Comparative Example 9

A separation membrane was obtained in the same manner as in Example 33,except that the composition of the resin composition for melt spinningand the production conditions were each changed as shown in Table 5. Thephysical properties of the resulting separation membrane and theperformances of the separation membrane module are shown in Table 5. Theresin composition in Table 5 are shown in Table 7.

Example 39

91 parts by weight of the copolymer of polyethylene terephthalate with5-sodium sulfoisophthalic acid (F1), 1 part by weight of polyethyleneglycol (Mw: 1,000) (B3), and 8 parts by weight of polyethylene glycol(Mw: 100,000) (C7) were melt-kneaded in a twin-screw extruder at 270° C.and then pelletized to obtain a resin composition for melt spinning. Thepellets were dried in vacuum at 100° C. for 15 hours.

The dried pellets were fed into a twin-screw extruder and melted at 270°C. Thereafter, the resultant was introduced into a melt spinning packadjusted to 270° C. in spinning temperature, and spun downwards underconditions of a discharge rate of 60 g/min from a spinneret having 72spinneret holes (a type of forming one discharge hole by arranging 3arcuate slit parts, discharge hole radius: 0.60 mm, pitch between slits:0.10 mm, slit width: 0.08 mm). The spun hollow fibers were introducedinto a cooling apparatus (chimney) so that a distance H from a lowersurface of the spinneret to an upper end of the cooling apparatus was 30mm, cooled by cooling air at 25° C. and an air velocity of 1.5 m/sec,subjected to application of an oiling agent, bundled, and then wound bya winder at a draft ratio of 400. The physical properties of the thusobtained separation membrane are shown in Table 5.

A module including this separation membrane was prepared and immersed ina 10 wt % aqueous solution of isopropyl alcohol for 1 hour, therebyperforming hydrophilization. Thereafter, the membrane permeation fluxand separation performance were evaluated. The results are shown inTable 5. The resin composition in Table 5 is shown in Table 7.

Example 40

A separation membrane was obtained in the same manner as in Example 37,except that the composition of the resin composition for melt spinningand the production conditions were each changed as shown in Table 5. Thephysical properties of the resulting separation membrane and theperformances of the separation membrane module are shown in Table 5. Theresin composition in Table 5 are shown in Table 7.

Comparative Example 10

A separation membrane was obtained in the same manner as in Example 37,except that the composition of the resin composition for melt spinningand the production conditions were each changed as shown in Table 5. Thephysical properties of the resulting separation membrane and theperformances of the separation membrane module are shown in Table 5. Theresin composition in Table 5 are shown in Table 7.

Example 41

The raw materials were melt-kneaded in a twin-screw extruder at 220° C.so as to have a composition described in the [First layer] resincomposition of Table 6, thereby obtaining a resin composition for thefirst layer. In addition, the raw materials were melt-kneaded in atwin-screw extruder at 220° C. so as to have a composition described inthe [Second layer] resin composition of Table 6, thereby obtaining acomplexing resin composition for the second layer. The resin compositionfor the first layer and the complexing resin composition for the secondlayer were each adjusted by a separate gear pump, such that extrusionamounts of the resin composition for the first layer and the complexingresin composition for the second layer were 2.4 g/min and 7.2 g/min,respectively, followed by feeding into a spinning pack.

Subsequently, the resin compositions were introduced into a spinnerethaving a multi-tube nozzle in which a channel of gas is arranged in thecenter thereof, such that the first layer formed an outer layer, and thesecond layer formed an inner layer, followed by complexing within thespinneret. Thereafter, the resultant was spun downwards from spinneretholes (outer diameter: 4.6 mm, inner diameter: 3.7 mm, slit width: 0.45mm, the number of holes: 4). The spun hollow fibers were introduced intoa cooling apparatus (chimney) so that a distance L from a lower surfaceof the spinneret to an upper end of the cooling apparatus was 50 mm,cooled by cooling air at 25° C. and an air velocity of 1.5 m/sec,subjected to application of an oiling agent, bundled, and then wound bya winder at a draft ratio of 400.

The physical properties of the resulting composite hollow fiber membraneare shown in Table 6. A module including this separation membrane wasprepared and immersed in a 10 wt % aqueous solution of isopropyl alcoholfor 1 hour, thereby performing hydrophilization. Thereafter, themembrane permeation flux and separation performance were evaluated. Theresults are shown in Table 6. The resin composition in Table 6 is shownin Table 7.

Example 42

The raw materials were melt-kneaded in a twin-screw extruder at 220° C.so as to have a composition described in the [First layer] resincomposition of Table 6, and then pelletized to obtain a resincomposition for the first layer. The pellets were dried in vacuum at 80°C. for 8 hours.

In addition, the raw materials were melt-kneaded in a twin-screwextruder at 220° C. so as to have a composition described in the [Secondlayer] resin composition of Table 6, and then pelletized to obtain acomplexing resin composition for the second layer. The pellets weredried in vacuum at 80° C. for 8 hours.

The dried pellets of the resin composition for the first layer and thedried pellets of the complexing resin composition for the second layerwere each fed into a separate twin-screw extruder and melt-kneaded at235° C. and then adjusted by a separate gear pump, such that extrusionamounts of the resin composition for the first layer and the complexingresin composition for the second layer were 2.4 g/min and 7.2 g/min,respectively. Subsequently, the resin compositions were introduced intoa spinneret having a multi-tube nozzle in which a channel of gas isarranged in the center thereof, such that the first layer formed anouter layer, and the second layer formed an inner layer, followed bycomplexing within the spinneret.

Thereafter, the resultant was spun downwards from spinneret holes (outerdiameter: 4.6 mm, inner diameter: 3.7 mm, slit width: 0.45 mm, thenumber of holes: 4). The spun hollow fibers were introduced into acooling apparatus (chimney) so that a distance L from a lower surface ofthe spinneret to an upper end of the cooling apparatus was 50 mm, cooledby cooling air at 25° C. and an air velocity of 1.5 m/sec, subjected toapplication of an oiling agent, bundled, and then wound by a winder at adraft ratio of 400. The physical properties of the resulting compositehollow fiber membrane are shown in Table 6. A module including thisseparation membrane was prepared and immersed in a 10 wt % aqueoussolution of isopropyl alcohol for 1 hour, thereby performinghydrophilization. Thereafter, the membrane permeation flux andseparation performance were evaluated. The results are shown in Table 6.The resin composition in Table 6 is shown in Table 7.

Example 43

The raw materials were melt-kneaded in a twin-screw extruder at 270° C.so as to have a composition described in the [First layer] resincomposition of Table 6, and then pelletized to obtain a resincomposition for the first layer. The pellets were dried in vacuum at 80°C. for 15 hours.

In addition, the raw materials were melt-kneaded in a twin-screwextruder at 270° C. so as to have a composition described in the [Secondlayer] resin composition of Table 6, and then pelletized to obtain acomplexing resin composition for the second layer. The pellets weredried in vacuum at 80° C. for 15 hours.

The dried pellets of the resin composition for the first layer and thedried pellets of the complexing resin composition for the second layerwere each fed into a separate twin-screw extruder and melt-kneaded at240° C. and then adjusted by a separate gear pump, such that extrusionamounts of the resin composition for the first layer and the complexingresin composition for the second layer were 2.4 g/min and 7.2 g/min,respectively.

Subsequently, the resin compositions were introduced into a spinnerethaving a multi-tube nozzle in which a channel of gas is arranged in thecenter thereof, such that the first layer formed an outer layer, and thesecond layer formed an inner layer, followed by complexing within thespinneret. Thereafter, the resultant was spun downwards from spinneretholes (outer diameter: 4.6 mm, inner diameter: 3.7 mm, slit width: 0.45mm, the number of holes: 4).

The spun hollow fibers were introduced into a cooling apparatus(chimney) so that a distance L from a lower surface of the spinneret toan upper end of the cooling apparatus was 50 mm, cooled by cooling airat 25° C. and an air velocity of 1.5 m/sec, subjected to application ofan oiling agent, bundled, and then wound by a winder at a draft ratio of400.

The physical properties of the resulting composite hollow fiber membraneare shown in Table 6. A module including this separation membrane wasprepared and immersed in a 10 wt % aqueous solution of isopropyl alcoholfor 1 hour, thereby performing hydrophilization. Thereafter, themembrane permeation flux and separation performance were evaluated. Theresults are shown in Table 6. The resin composition in Table 6 is shownin Table 7.

TABLE 1 (a) Dimensions of void Section Length (L₁) Depth (D₁) Width (W₁)A 150 nm or more 30 nm or more and 300 nm or less 10 nm or more and 300nm or less B 50 nm or more 20 nm or more and 400 nm or less 7 nm or moreand 400 nm or less C 30 nm or more 10 nm or more and 500 nm or less 5 nmor more and 500 nm or less D Less than 30 nm Less than 10 nm, or morethan 500 nm Less than 5 nm, or more than 500 nm (b) Dimensions of grooveSection Length (L₂) Depth (D₂) Width (W₂) A 50 nm or more 10 nm or moreand 300 nm or less 10 nm or more and 300 nm or less B 40 nm or more 7 nmor more and 400 nm or less 7 nm or more and 400 nm or less C 30 nm ormore 5 nm or more and 500 nm or less 5 nm or more and 500 nm or less DLess than 30 nm Less than 5 nm, or more than 500 nm Less than 5 nm, ormore than 500 nm

TABLE 2 Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple4 ple 5 ple 6 ple 7 Resin Cellulose ester (A) Kind A1 A1 A1 A1 A1 A1 A1composition wt % 82 82 82 82 82 82 82 for melt Plasticizer (B) Kind B1B1 B1 B1 B1 B1 B1 spinning wt % 15.9 13.9 13.9 13.9 17.4 16.9 15.9Additive (C) Kind C1 C1 C2 C3 C3 C3 C3 wt % 2.0 4.0 4.0 4.0 0.5 1.0 2.0Antioxidant (D) Kind D1 D1 D1 D1 D1 D1 D1 wt % 0.1 0.1 0.1 0.1 0.1 0.10.1 Production Spinning temperature ° C. 235 235 235 235 225 225 225conditions Draft ratio — 200 200 200 200 400 400 400 Physical Void shape(L₁) — B A A A B A A properties Void shape (D₁) — B A A A B A A andmembrane Void shape (W₁) — B B A A C B A performance Void L₁/D₁ — 2 5 710 3 7 9 Occupation area % 0.7 2.4 2.1 4.8 1.1 2.6 4.5 proportion ofvoid Groove shape (L₂) — B A A A B A A Groove shape (D₂) — B A A A B A AGroove shape (W₂) — C A A A C A A Groove L₂/W₂ — 2 5 6 8 2 5 7Occupation area % 0.6 1.2 1.0 3.6 0.5 1.0 3.4 proportion of groove Outerdiameter μm 49 50 55 57 71 71 67 Percentage of % 24 34 27 29 40 39 39hollowness Membrane permeation L/m²/day 3.4 5.3 4.3 13.9 3.5 4.8 14.5flux Salt rejection % 94.5 93.5 88.9 92.5 95.8 93.3 91.3 Tensileelasticity MPa 2,591 2,388 2,264 2,372 3,030 2,990 2,906 Tensilestrength MPa 113 102 102 101 131 129 128 Exam- Exam- Exam- Exam- Exam-Exam- Exam- ple 8 ple 9 ple 10 ple 11 ple 12 ple 13 ple 14 ResinCellulose ester (A) Kind A1 A1 A1 A1 A1 A1 A1 composition wt % 82 82 8274.9 74.9 82 82 for melt Plasticizer (B) Kind B1 B1 B1 B1 B1 B1 B1spinning wt % 13.9 11.9 9.9 15 10 13.9 13.9 Additive (C) Kind C3 C3 C3C3 C3 C4 C5 wt % 4.0 6.0 8.0 10.0 15.0 4.0 4.0 Antioxidant (D) Kind D1D1 D1 D1 D1 D1 D1 wt % 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Production Spinningtemperature ° C. 225 225 225 225 225 235 235 conditions Draft ratio —400 400 400 400 400 200 200 Physical Void shape (L₁) — A A A A A A Aproperties Void shape (D₁) — A A A A A A A and membrane Void shape (W₁)— A A A A A A A performance Void L₁/D₁ — 11 14 15 14 11 8 8 Occupationarea % 7.8 9.1 13.9 18.8 27.8 4.3 4.6 proportion of void Groove shape(L₂) — A A A A A A A Groove shape (D₂) — A A A A B A A Groove shape (W₂)— A A A A A A A Groove L₂/W₂ — 10 13 13 12 9 7 8 Occupation area % 7.28.4 13.1 16.3 19.6 3.2 3.6 proportion of groove Outer diameter μm 66 6157 53 52 48 49 Percentage of % 36 34 33 30 31 26 25 hollowness Membranepermeation L/m²/day 15.3 17.9 19.3 22.9 25.9 10.6 11.3 flux Saltrejection % 90.1 85.3 82.4 55.6 33.8 90.6 89.2 Tensile elasticity MPa2,786 2,573 2,390 2,195 1,995 2,377 2,210 Tensile strength MPa 120 111101 94 92 100 103

TABLE 3 Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 15 ple 16 ple 17ple 18 ple 19 ple 20 ple 21 Resin Cellulose ester (A) Kind A1 A1 A1 A1A1 A1 A1 composition wt % 82 82 82 82 82 82 82 for melt Plasticizer (B)Kind B1 B1 B1 B1 B1 B1 B1 spinning wt % 13.9 13.9 13.9 15.9 9.9 17.416.9 Additive (C) Kind C6 C6 C6 C6 C6 C6 C6 wt % 4.0 4.0 4.0 2.0 8.0 0.51.0 Antioxidant (D) Kind D1 D1 D1 D1 D1 D1 D1 wt % 0.1 0.1 0.1 0.1 0.10.1 0.1 Production Spinning temperature ° C. 235 235 235 235 235 225 225conditions Draft ratio — 200 400 750 400 750 400 400 Physical Void shape(L₁) — A A A A A B A properties Void shape (D₁) — A A A A A B A andmembrane Void shape (W₁) — A A A A B C B performance Void L₁/D₁ — 7 1113 3 18 3 5 Occupation area % 2.4 4.5 4.0 1.3 11.3 1.2 2.5 proportion ofvoid Groove shape (L₂) — A A A B A B A Groove shape (D₂) — A A A B A B AGroove shape (W₂) — A A A B A C A Groove L₂/W₂ — 5 9 8 3 17 2 4Occupation area % 1.2 3.7 3.2 0.9 10.4 0.5 1.1 proportion of grooveOuter diameter μm 52 39 37 38 50 68 69 Percentage of % 35 33 32 29 33 3839 hollowness Membrane permeation L/m²/day 5.0 11.7 13.4 3.6 15.9 3.25.3 flux Salt rejection % 92.6 92.0 90.5 92.2 83.3 96.2 92.3 Tensileelasticity MPa 2,315 2,301 2,785 2,611 2,689 3,061 2,998 Tensilestrength MPa 102 100 102 108 97 134 131 Exam- Exam- Exam- Exam- Exam-Exam- Exam- ple 22 ple 23 ple 24 ple 25 ple 26 ple 27 ple 28 ResinCellulose ester (A) Kind A1 A1 A1 A1 A1 A1 A1 composition wt % 82 82 8282 74.9 74.9 82 for melt Plasticizer (B) Kind B1 B1 B1 B1 B1 B1 B1spinning wt % 15.9 13.9 11.9 9.9 15 10 15.9 Additive (C) Kind C6 C6 C6C6 C6 C6 C7 wt % 2.0 4.0 6.0 8.0 10.0 15.0 2.0 Antioxidant (D) Kind D1D1 D1 D1 D1 D1 D1 wt % 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Production Spinningtemperature ° C. 225 225 225 225 225 225 235 conditions Draft ratio —400 400 400 400 400 400 200 Physical Void shape (L₁) — A A A A A A Aproperties Void shape (D₁) — A A A A A A A and membrane Void shape (W₁)— A A A A A A A performance Void L₁/D₁ — 8 13 16 19 16 11 9 Occupationarea % 4.6 7.4 9.8 14.3 18.2 28.3 2.6 proportion of void Groove shape(L₂) — A A A A A A A Groove shape (D₂) — A A A A A B A Groove shape (W₂)— A A A A A A A Groove L₂/W₂ — 8 11 13 18 16 10 8 Occupation area % 3.36.4 9.2 13.2 17.4 17.4 1.8 proportion of groove Outer diameter μm 67 6160 58 52 52 49 Percentage of % 38 35 33 32 31 31 34 hollowness Membranepermeation L/m²/day 11.3 14.8 16.7 17.9 23.7 29.0 5.1 flux Saltrejection % 90.5 92.3 85.6 80.9 65.5 39.6 92.3 Tensile elasticity MPa2,815 2,633 2,483 2,235 2,165 1,965 2,521 Tensile strength MPa 122 113108 102 93 90 110 Exam- Exam- Exam- Exam- ple 29 ple 30 ple 31 ple 32Resin Cellulose ester (A) Kind A1 A1 A1 A1 composition wt % 82 82 82 82for melt Plasticizer (B) Kind B1 B1 B1 B1 spinning wt % 15.9 15.9 13.913.9 Additive (C) Kind C7 C7 C7 C8 wt % 2.0 2.0 4.0 4.0 Antioxidant (D)Kind D1 D1 D1 D1 wt % 0.1 0.1 0.1 0.1 Production Spinning temperature °C. 235 235 225 225 conditions Draft ratio — 400 750 400 400 PhysicalVoid shape (L₁) — A A A A properties Void shape (D₁) — A A A A andmembrane Void shape (W₁) — A A A A performance Void L₁/D₁ — 9 11 10 8Occupation area % 2.4 2.3 7.8 3.4 proportion of void Groove shape (L₂) —A A A A Groove shape (D) — A A A A Groove shape (W₂) — A A A A GrooveL₂/W₂ — 7 7 10 7 Occupation area % 1.9 1.8 7.3 6.2 proportion of grooveOuter diameter μm 37 32 36 37 Percentage of % 33 34 32 33 hollownessMembrane permeation L/m²/day 5.6 5.6 15.2 11.9 flux Salt rejection %90.2 86.0 83.3 86.5 Tensile elasticity MPa 2,696 3,004 2,539 2,463Tensile strength MPa 116 124 107 108

TABLE 4 Compar- Compar- Compar- Compar- Compar- Compar- Compar- ativeative ative ative ative ative ative Exam- Exam- Exam- Exam- Exam- Exam-Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 Resin Cellulose ester(A) Kind A1 A1 A2 A1 A1 A1 A1 composition wt % 82 70 82 74 82 82 70 formelt Plasticizer (B) Kind B1 B1 B1 B1 B1 B1 B1 spinning wt % 15.9 9.917.9 25.9 17.9 15.9 9.9 Additive (C) Kind C3 C3 — — — C6 C6 wt % 0.120.0 — — — 0.1 20.0 Antioxidant (D) Kind D1 D1 D1 D1 D1 D1 D1 wt % 0.10.1 0.1 0.1 0.1 0.1 0.1 Production Spinning temperature ° C. 225 225 260230 235 225 225 conditions Draft ratio — 400 400 400 400 400 400 400Physical Void shape (L₁) — — A — — — A properties Void shape (D₁) — — D— — — D and membrane Void shape (W₁) — — C — — — C performance VoidL₁/D₁ — — 1 — — — 1 Occupation area % — 38.1 0.0 0.0 — 36.7 proportionof void Groove shape (L₂) — — A — — — A Groove shape (D₂) — — C — — — CGroove shape (W₂) — — D — — — D Groove L₂/W₂ — — 2 — — — 2 Occupationarea % — 34.7 0.0 0.0 — 36.3 proportion of groove Outer diameter μm 6940 51 53 53 43 Percentage of % 38 24 37 39 38 26 hollowness Membranepermeation L/m²/day 1.8 46.0 2.5 1.0 2.6 36.6 flux Salt rejection % 97.810.9 93.0 98.7 98.2 2.1 Tensile elasticity MPa 3,061 1,389 1,586 3,0813,076 1,296 Tensile strength MPa 149 76 102 153 151 77

TABLE 5 Example 33 Example 34 Example 35 Example 36 Example 37 ResinPolyamide (E) or Kind E1 E1 E1 E2 E2 composition polyester (F) wt % 89.592.5 86.5 89.5 92.5 for melt Plasticizer (B) Kind B2 B2 B2 B2 B2spinning wt % 5 5 5 5 5 Additive (C) Kind C7 C7 C7 C7 C7 wt % 5.0 2.08.0 5.0 2.0 Antioxidant (D) Kind D2 D2 D2 D2 D2 wt % 0.5 0.5 0.5 0.5 0.5Production Spinning temperature ° C. 250 250 250 250 250 conditionsDraft ratio — 400 400 400 400 400 Physical Void shape (L₁) — A A A A Aproperties Void shape (D₁) — A A A A A and membrane Void shape (W₁) — AA A A A performance Void L₁/D₁ — 10 9 15 12 11 Occupation area % 6.9 4.212.6 7.3 5.1 proportion of void Groove shape (L₂) — A A A A A Grooveshape (D₂) — A A A A A Groove shape (W₂) — A A A A A Groove L₂/W₂ — 8 714 10 10 Occupation area % 6.5 3.6 13.5 6.6 4.3 proportion of grooveOuter diameter μm 46 53 43 49 48 Percentage of % 33 34 31 34 33hollowness Membrane permeation L/m²/day 13.3 11.3 18.4 14.3 12.4 fluxSalt rejection % 87.3 92.6 83.7 89.6 92.1 Tensile elasticity MPa 2,3122,891 2,150 2,256 2,963 Tensile strength MPa 109 125 102 107 128Comparative Comparative Example 38 Example 39 Example 40 Example 9Example 10 Resin Polyamide (E) or Kind E2 F1 F1 E1 F1 compositionpolyester (F) wt % 86.5 91 97 89.5 98.9 for melt Plasticizer (B) Kind B2B3 B3 B2 B3 spinning wt % 5 1 1 9.9 1 Additive (C) Kind C7 C7 C7 C7 C7wt % 8.0 8.0 2.0 0.1 0.1 Antioxidant (D) Kind D2 — — D2 — wt % 0.5 — —0.5 — Production Spinning temperature ° C. 250 270 270 250 270conditions Draft ratio — 400 400 400 400 400 Physical Void shape (L₁) —A A A — — properties Void shape (D₁) — A A A — — and membrane Void shape(W₁) — A A A — — performance Void L₁/D₁ — 16 11 8 — — Occupation area %11.8 7.5 3.8 — — proportion of void Groove shape (L₂) — A A A — — Grooveshape (D₂) — A A A — — Groove shape (W₂) — A A A — — Groove L₂/W₂ — 1110 7 — — Occupation area % 11.7 7.2 3.9 — — proportion of groove Outerdiameter μm 44 43 48 35 43 Percentage of % 31 31 36 30 31 hollownessMembrane permeation L/m²/day 18.7 17.8 12.8 2.3 2.8 flux Salt rejection% 85.8 83.6 87.5 89.5 84.6 Tensile elasticity MPa 2,262 2,460 2,7102,368 2,460 Tensile strength MPa 106 110 119 111 110

TABLE 6 Example Example Example 41 42 43 [Fist layer] Main componentKind A1 A1 E1 resin composition mass % 82 82 89.5 Plasticizer (B) KindB1 B1 B2 mass % 13.9 13.9 5 Plasticizer (C) Kind C3 C6 C7 mass % 4 4 5Antioxidant (D) Kind D1 D1 D2 mass % 0.1 0.1 0.5 [Second layer] Maincomponent Kind A1 A1 E1 complexing resin mass % 74 70 89.5 compositionPlasticizer (B) Kind B1 B1 B2 mass % 17.9 9.9 5 Plasticizer (C) Kind C3C6 C7 mass % 20 20 20 Antioxidant (D) Kind D1 D1 D2 mass % 0.1 0.1 0.5Production Spinning temperature ° C. 225 225 250 conditions Draft ratio— 400 400 400 Structure of Outer diameter μm 76 64 70 composite hollowPercentage of hollowness % 37 32 35 fiber membrane Void shape (L₁) oflayer (A) — A A A Void shape (D₁) of layer (A) — A A A Void shape (W₁)of layer (A) — A A A Void L₁/D₁ of layer (A) — 11 12 11 Occupation areaproportion (H) of % 7.6 7.6 7.2 void of layer (A) Groove shape (L₂) oflayer (A) — A A A Groove shape (D₂) of layer (A) — A A A Groove shape(W₂) of layer (A) — A A A Groove L₂/W₂ of layer (A) — 10 11 9 Occupationarea proportion of groove % 7.3 6.5 6.5 Thickness f layer (A) μm 4.2 3.31.8 Rate of hole area H_(B) of layer (B) % 38.9 33.2 33.9 Physicalproperties Membrane permeation flux L/m²/day 21.2 20.2 23.3 of compositeSalt rejection % 87.6 92.2 83.5 hollow fiber Tensile elasticity MPa1,570 1,808 1,708 membrane Tensile strength MPa 83 90 86

TABLE 7 Classification No. Compound Cellulose ester A1 Cellulose acetatepropionate (A) A2 Cellulose acetate Plasticizer (B) B1 Polyethyleneglycol (Mw: 600) B2 Polyvinylpyrrolidone (K30) B3 Polyethylene glycol(Mw: 1,000) Additive (C) C1 Glycerin C2 Diglycerin C3 Diglycerin oleateC4 Octyl phthalate C5 Dioctyl adipate C6 Polyethylene glycol (Mw: 8300)C7 Polyethylene glycol (Mw: 100,000) C8 Polyethylene glycol (Mw:300,000) Antioxidant D1Bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol (D) diphosphite D2Irganox 1098 Polyamide (E) E1 Nylon 6 (Nylon 6 resin “Amilan”,manufactured by Toray Industries, Inc.) E2 Nylon 66 (Nylon 66 resin“Amilan”, manufactured by Toray Industries, Inc.) Polyester (F) F1Copolymer of polyethylene terephthalate with 5-sodium sulfoisophthalicacid

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof. It is to benoted that the present application is based on a Japanese patentapplication filed on Mar. 31, 2015 (Japanese Patent Application No.2015-072327), the entireties of which are incorporated by reference.

INDUSTRIAL APPLICABILITY

The present invention provides a separation membrane having excellentseparation performance and permeation performance, having high membranestrength, and mainly including a cellulose-based resin. The separationmembrane of the present invention can be used for water treatmentmembranes for producing industrial water, drinking water and the likefrom seawater, brackish water, sewage water, waste water and the like,medical membranes for artificial kidneys, plasma separation and thelike, membranes for food-beverage industry such as fruit juiceconcentration, gas separation membranes for separating exhaust gas,carbonic acid gas, and the like, membranes for electronic industry suchas fuel cell separators, and the like. The above-mentioned watertreatment membrane can be preferably used for microfiltration membranes,ultrafiltration membranes, nanofiltration membranes, reverse osmosismembranes, forward osmosis membranes, and the like.

The invention claimed is:
 1. A separation membrane comprising at least afirst layer, wherein the first layer comprises, as a main componentthereof, at least one compound selected from the group consisting of acellulose ester, a polyamide, and a polyester, the first layer has, inan interior thereof, a plurality of voids each having a depth (D₁) of 10nm or more and 500 nm or less, a length (L₁) of 30 nm or more, and aratio L₁/D₁ of the length to the depth in a range of 4 or more, and theseparation membrane has a tensile elasticity of 1,800 to 6,500 MPa. 2.The separation membrane according to claim 1, wherein a lengthwisedirection of the void follows a lengthwise direction of the separationmembrane.
 3. The separation membrane according to claim 1, wherein, whena projected area of a cross-section of the separation membrane isdefined as S₁₀, and an occupation area of the voids is defined as S₁, anoccupancy rate of the voids in the cross-section, expressed by{(S₁/S₁₀)×100}, is 0.5% or more and 30% or less.
 4. The separationmembrane according to claim 1, having, on at least one surface thereof,a plurality of grooves each having a length (L₂) of 30 nm or more, awidth (W₂) of 5 nm or more and 500 nm or less, and a ratio L₂/W₂ of thelength to the width in a range of 2 or more.
 5. The separation membraneaccording to claim 4, wherein a lengthwise direction of the groovesfollow a lengthwise direction of the separation membrane.
 6. Theseparation membrane according to claim 1, further comprising a secondlayer that has a rate of hole area which is different from an occupancyrate of the voids of the first layer.
 7. The separation membraneaccording to claim 6, wherein a lengthwise direction of the voids followa lengthwise direction of the separation membrane in the first layer. 8.The separation membrane according to claim 6, wherein, when a projectedarea of a cross-section of the separation membrane is defined as S₁₀,and an occupation area of the voids is defined as S₁, an occupancy rateof the voids in the cross-section, as expressed by {(S₁/S₁₀)×100}, is0.5% or more and 30% or less in the first layer.
 9. The separationmembrane according to claim 6, having, on at least one surface of thefirst layer thereof, a plurality of grooves each having a length (L₂) of30 nm or more, a width (W₂) of 5 nm or more and 500 nm or less, and aratio L₂/W₂ of the length to the width in a range of 2 or more.
 10. Theseparation membrane according to claim 9, wherein a lengthwise directionof the grooves follow a lengthwise direction of the separation membranein the first layer.
 11. The separation membrane according to claim 9,wherein, when a projected area of a surface of the separation membraneis defined as S₂₀, and an occupation area of the grooves is defined asS₂, an occupancy rate of the grooves in the surface, as expressed by{(S₂/S₂₀)×100}, is 0.5% or more and 20% or less in the first layer. 12.The separation membrane according to claim 6, wherein an occupancy rateV_(A) of voids in a cross-section of the first layer and a rate of holearea H_(B) of the second layer satisfy a relation: V_(A)<H_(B); andwherein the rate of hole area H_(B) corresponds to (average pore aream²)×(fine pore density per m²)×100.
 13. The separation membraneaccording to claim 1, wherein the separation membrane has a shape of ahollow fiber.
 14. The separation membrane according to claim 13, whereinthe hollow fiber has an outer diameter of 20 μm to 400 μm.
 15. Theseparation membrane according to claim 1, wherein the separationmembrane comprises, as the main component thereof, the cellulose ester,and the separation membrane comprises, as the cellulose ester, at leastone of cellulose acetate propionate and cellulose acetate butyrate. 16.The separation membrane according to claim 1, wherein the separationmembrane is at least one selected from the group consisting of ananofiltration membrane, a reverse osmosis membrane, a forward osmosismembrane, and a gas separation membrane.
 17. A membrane modulecomprising the separation membrane according to claim 1.